DRAFT Guide to the Secure Configuration of Red Hat Enterprise Linux 6

Rules lacking CCEs

encrypt_partitions security_patches_up_to_date aide_build_database install_hids install_antivirus file_permissions_library_dirs file_ownership_library_dirs file_permissions_binary_dirs file_ownership_binary_dirs selinux_confinement_of_daemons no_root_webbrowsing root_path_default gid_passwd_group_same account_disable_post_pw_expiration account_unique_name account_temp_expire_date max_concurrent_login_sessions disable_ctrlaltdel_reboot smartcard_auth iptables_icmp_disabled iptables_log_and_drop_suspicious install_openswan disable_logwatch_for_logserver configure_auditd_num_logs configure_auditd_max_log_file configure_auditd_audispd disable_anacron sshd_limit_user_access avahi_ip_only avahi_check_ttl avahi_prevent_port_sharing avahi_disable_publishing ntpd_specify_multiple_servers package_sendmail_removed iptables_smtp_enabled postfix_logging postfix_install_ssl_cert postfix_server_denial_of_service postfix_server_banner postfix_server_mail_relay_set_trusted_networks postfix_server_mail_relay_for_trusted_networks postfix_server_mail_relay_smtp_auth_for_untrusted_networks postfix_server_mail_relay_require_tls_for_smtp_auth ldap_server_config_bdb_file_security ldap_server_config_olcrootpw ldap_server_config_olcaccess iptables_ldap_enabled ldap_server_config_logging dns_server_disable_zone_transfers dns_server_authenticate_zone_transfers ftp_home_partition httpd_servertokens_prod httpd_serversignature_off httpd_digest_authentication httpd_mod_rewrite httpd_ldap_support httpd_server_side_includes httpd_mime_magic httpd_webdav httpd_server_activity_status httpd_server_configuration_display httpd_url_correction httpd_proxy_support httpd_cache_support httpd_cgi_support httpd_restrict_root_directory httpd_restrict_web_directory httpd_restrict_critical_directories httpd_limit_available_methods httpd_install_mod_ssl httpd_install_mod_security httpd_conf_dir_permissions httpd_conf_files_permissions dovecot_enable_ssl dovecot_configure_ssl_cert dovecot_configure_ssl_key smb_server_disable_root smb_set_auth_levels snmpd_use_newer_protocol snmpd_not_default_password met_inherently_generic met_inherently_auditing met_inherently_nonselected unmet_nonfinding_nonselected_scope unmet_finding_nonselected unmet_nonfinding_scope update_process

Rules lacking 800-53 references

no_root_webbrowsing root_path_default gid_passwd_group_same account_unique_name password_require_consecrepeat max_concurrent_login_sessions disable_ctrlaltdel_reboot smartcard_auth configure_logwatch_hostlimit configure_logwatch_splithosts disable_logwatch_for_logserver ssh_server_disabled ssh_server_iptables_exception sshd_limit_user_access sshd_set_idle_timeout sshd_set_keepalive sshd_disable_rhosts disable_host_auth sshd_disable_empty_passwords sshd_enable_warning_banner sshd_do_not_permit_user_env disable_xwindows_with_runlevel packagegroup_xwindows_remove service_nfslock_disabled service_rpcgssd_disabled service_rpcidmapd_disabled service_netfs_disabled service_rpcbind_disabled nfs_fixed_lockd_tcp_port nfs_fixed_lockd_udp_port nfs_fixed_statd_port nfs_fixed_mountd_port service_nfs_disabled service_rpcsvcgssd_disabled use_nodev_option_on_nfs_mounts use_nosuid_option_on_nfs_mounts use_root_squashing_all_exports restrict_nfs_clients_to_privileged_ports no_insecure_locks_exports dns_server_disable_dynamic_updates ftp_log_transactions ftp_present_banner ftp_restrict_to_anon ftp_disable_uploads ftp_home_partition httpd_digest_authentication httpd_mod_rewrite httpd_ldap_support httpd_server_side_includes httpd_mime_magic httpd_webdav httpd_server_activity_status httpd_server_configuration_display httpd_url_correction httpd_proxy_support httpd_cache_support httpd_cgi_support httpd_restrict_root_directory httpd_restrict_web_directory httpd_restrict_critical_directories httpd_limit_available_methods httpd_install_mod_ssl httpd_install_mod_security httpd_conf_dir_permissions disable_dovecot uninstall_dovecot dovecot_support_necessary_protocols dovecot_enable_ssl dovecot_configure_ssl_cert dovecot_configure_ssl_key dovecot_disable_plaintext_auth disable_smb_server smb_server_disable_root smb_set_auth_levels require_smb_client_signing require_smb_client_signing_mount.cifs disable_squid uninstall_squid disable_snmpd uninstall_net-snmp snmpd_use_newer_protocol snmpd_not_default_password met_inherently_generic met_inherently_auditing met_inherently_nonselected unmet_nonfinding_nonselected_scope unmet_finding_nonselected unmet_nonfinding_scope update_process

Details

Groups and Rules
SeverityCCEDescriptionSettingRationaleImpactCategoryReferences
Introduction The purpose of this guidance is to provide security configuration recommendations and baselines for the Red Hat Enterprise Linux (RHEL) 6 operating system. The guidance provided here should be applicable to all variants (Desktop, Server, Advanced Platform) of the product. Recommended settings for the basic operating system are provided, as well as for many network services that the system can provide to other systems. The guide is intended for system administrators. Readers are assumed to possess basic system administration skills for Unix-like systems, as well as some familiarity with Red Hat's documentation and administration conventions. Some instructions within this guide are complex. All directions should be followed completely and with understanding of their effects in order to avoid serious adverse effects on the system and its security.
General Principles The following general principles motivate much of the advice in this guide and should also influence any configuration decisions that are not explicitly covered.
Encrypt Transmitted Data Whenever Possible Data transmitted over a network, whether wired or wireless, is susceptible to passive monitoring. Whenever practical solutions for encrypting such data exist, they should be applied. Even if data is expected to be transmitted only over a local network, it should still be encrypted. Encrypting authentication data, such as passwords, is particularly important. Networks of RHEL6 machines can and should be configured so that no unencrypted authentication data is ever transmitted between machines.
Minimize Software to Minimize Vulnerability The simplest way to avoid vulnerabilities in software is to avoid installing that software. On RHEL, the RPM Package Manager (originally Red Hat Package Manager, abbreviated RPM) allows for careful management of the set of software packages installed on a system. Installed software contributes to system vulnerability in several ways. Packages that include setuid programs may provide local attackers a potential path to privilege escalation. Packages that include network services may give this opportunity to network-based attackers. Packages that include programs which are predictably executed by local users (e.g. after graphical login) may provide opportunities for trojan horses or other attack code to be run undetected. The number of software packages installed on a system can almost always be significantly pruned to include only the software for which there is an environmental or operational need.
Run Different Network Services on Separate Systems Whenever possible, a server should be dedicated to serving exactly one network service. This limits the number of other services that can be compromised in the event that an attacker is able to successfully exploit a software flaw in one network service.
Configure Security Tools to Improve System Robustness Several tools exist which can be effectively used to improve a system's resistance to and detection of unknown attacks. These tools can improve robustness against attack at the cost of relatively little configuration effort. In particular, this guide recommends and discusses the use of Iptables for host-based firewalling, SELinux for protection against vulnerable services, and a logging and auditing infrastructure for detection of problems.
Least Privilege Grant the least privilege necessary for user accounts and software to perform tasks. For example, sudo can be implemented to limit authorization to super user accounts on the system only to designated personnel. Another example is to limit logins on server systems to only those administrators who need to log into them in order to perform administration tasks. Using SELinux also follows the principle of least privilege: SELinux policy can confine software to perform only actions on the system that are specifically allowed. This can be far more restrictive than the actions permissible by the traditional Unix permissions model.
How to Use This Guide Readers should heed the following points when using the guide.
Read Sections Completely and in Order Each section may build on information and recommendations discussed in prior sections. Each section should be read and understood completely; instructions should never be blindly applied. Relevant discussion will occur after instructions for an action. The system-level configuration guidance in Chapter 2 must be applied to all machines. The guidance for individual services in Chapter 3 must be considered for all machines as well: apply the guidance if the machine is either a server or a client for that service, and ensure that the service is disabled according to the instructions provided if the machine is neither a server nor a client.
Test in Non-Production Environment This guidance should always be tested in a non-production environment before deployment. This test environment should simulate the setup in which the system will be deployed as closely as possible.
Root Shell Environment Assumed Most of the actions listed in this document are written with the assumption that they will be executed by the root user running the /bin/bash shell. Commands preceded with a hash mark (#) assume that the administrator will execute the commands as root, i.e. apply the command via sudo whenever possible, or use su to gain root privileges if sudo cannot be used. Commands which can be executed as a non-root user are are preceded by a dollar sign ($) prompt.
Formatting Conventions Commands intended for shell execution, as well as configuration file text, are featured in a monospace font. Italics are used to indicate instances where the system administrator must substitute the appropriate information into a command or configuration file.
Reboot Required A system reboot is implicitly required after some actions in order to complete the reconfiguration of the system. In many cases, the changes will not take effect until a reboot is performed. In order to ensure that changes are applied properly and to test functionality, always reboot the system after applying a set of recommendations from this guide.
System Settings
Installing and Maintaining SoftwareThe following sections contain information on security-relevant choices during the initial operating system installation process and the setup of software updates.
Disk PartitioningTo ensure separation and protection of data there are top-level system directories which should be placed on their own physical partition or logical volume. The installer's default partitioning scheme creates separate logical volumes for /, /boot, and swap. If starting with any of the default layouts, check the box to "Review and modify partitioning." This allows for the easy creation of additional logical volumes inside the volume group already created, though it may require making /'s logical volume smaller to create space. In general, using logical volumes is preferable to using partitions because they can be more easily adjusted later.If creating a custom layout, create the partitions mentioned in the previous paragraph (which the installer will require anyway), as well as separate ones described in the following sections. If a system has already been installed, and the default partitioning scheme was used, it is possible but nontrivial to modify it to create separate logical volumes for the directories listed above. The Logical Volume Manager (LVM) makes this possible. See the LVM HOWTO at http://tldp.org/HOWTO/LVM-HOWTO/ for more detailed information on LVM.
lowCCE-26435-8Ensure /tmp Located On Separate Partition The /tmp directory is a world-writable directory used for temporary file storage. Ensure it has its own partition or logical volume at installation time, or migrate it using LVM. The /tmp partition is used as temporary storage by many programs. Placing /tmp in its own partition enables the setting of more restrictive mount options, which can help protect programs which use it. Stinging Nettle Charity
NIST SP800-53
lowCCE-26639-5Ensure /var Located On Separate PartitionThe /var directory is used by daemons and other system services to store frequently-changing data. Ensure that /var has its own partition or logical volume at installation time, or migrate it using LVM. Ensuring that /var is mounted on its own partition enables the setting of more restrictive mount options. This helps protect system services such as daemons or other programs which use it. It is not uncommon for the /var directory to contain world-writable directories, installed by other software packages. Sore Throat Kindness
NIST SP800-53
lowCCE-26215-4Ensure /var/log Located On Separate Partition System logs are stored in the /var/log directory. Ensure that it has its own partition or logical volume at installation time, or migrate it using LVM. Placing /var/log in its own partition enables better separation between log files and other files in /var/. Gallstones Temperance
NIST SP800-53 AU-9
DISA CCI 1208
lowCCE-26436-6Ensure /var/log/audit Located On Separate Partition Audit logs are stored in the /var/log/audit directory. Ensure that it has its own partition or logical volume at installation time, or migrate it later using LVM. Make absolutely certain that it is large enough to store all audit logs that will be created by the auditing daemon. Placing /var/log/audit in its own partition enables better separation between audit files and other files, and helps ensure that auditing cannot be halted due to the partition running out of space. Dehydration Kindness
NIST SP800-53 AU-9
DISA CCI 137, 138, 1208
lowCCE-26557-9Ensure /home Located On Separate Partition If user home directories will be stored locally, create a separate partition for /home at installation time (or migrate it later using LVM). If /home will be mounted from another system such as an NFS server, then creating a separate partition is not necessary at installation time, and the mountpoint can instead be configured later. Ensuring that /home is mounted on its own partition enables the setting of more restrictive mount options, and also helps ensure that users cannot trivially fill partitions used for log or audit data storage. Cramps Diligence
NIST SP800-53
lownsEncrypt Partitions Red Hat Enterprise Linux 6 natively supports partition encryption through the Linux Unified Key Setup-on-disk-format (LUKS) technology. The easiest way to encrypt a partition is during installation time. For manual installations, select the Encrypt checkbox during partition creation to encrypt the partition. When this option is selected the system will prompt for a passphrase to use in decrypting the partition. The passphrase will subsequently need to be entered manually every time the system boots. For automated/unattended installations, it is possible to use Kickstart by adding the --encrypted and --passphrase= options to the definition of each partition to be encrypted. For example, the following line would encrypt the root partition: part / --fstype=ext3 --size=100 --onpart=hda1 --encrypted --passphrase=PASSPHRASE Any PASSPHRASE is stored in the Kickstart in plaintext, and the Kickstart must then be protected accordingly. Omitting the --passphrase= option from the partition definition will cause the installer to pause and interactively ask for the passphrase during installation. Detailed information on encrypting partitions using LUKS can be found on the Red Had Documentation web site: https://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Security_Guide/sect-Security_Guide-LUKS_Disk_Encryption.html The risk of a system's physical compromise, particularly mobile systems such as laptops, places its data at risk of compromise. Encrypting this data mitigates the risk of its loss if the system is lost. Hiccups Patience
NIST SP800-53 SC-13, SC-28
DISA CCI 1019, 1199, 1200
Updating SoftwareThe yum command line tool is used to install and update software packages. The system also provides two graphical package managers, pirut and pup. The pirut tool is a graphical front-end for yum that allows users to install and update packages while pup is a simple update tool for packages that are already installed. In the Applications menu, pirut is labeled Add/Remove Software and pup is labeled Software Updater. Red Hat Enterprise Linux systems contain an embedded Installed Software Catalog, or "RPM Database," which records metadata of installed packages. The yum, pirut, and pup tools interface with the Installed Software Catalog to ensure all system metadata is accurate with regard to installed software and security patches, and for this reason, their use is strongly encouraged.
highCCE-26506-6Ensure Red Hat GPG Key Installed To ensure the system can cryptographically verify base software packages come from Red Hat (and to connect to the Red Hat Network to receive them if desired), the Red Hat GPG key must properly be installed. To ensure the GPG key is installed, run: # rhn_register This key is necessary to cryptographically verify packages are from Red Hat. Diarrhea Diligence
NIST SP800-53 SI-7
DISA CCI 351
highCCE-26709-6Ensure gpgcheck Enabled In Main Yum ConfigurationThe gpgcheck option should be used to ensure checking of an RPM package's signature always occurs prior to its installation. To configure yum to check package signatures before installing them, ensure the following line appears in /etc/yum.conf in the [main] section: gpgcheck=1 Ensuring the validity of packages' cryptographic signatures prior to installation ensures the provenance of the software and protects against malicious tampering. Tooth Ache Humility
NIST SP800-53 SI-7
DISA CCI 352, 663
highCCE-26647-8Ensure gpgcheck Enabled For All Yum Package RepositoriesTo ensure signature checking is not disabled for any repos, remove any lines from files in /etc/yum.repos.d of the form: gpgcheck=0 Ensuring all packages' cryptographic signatures are valid prior to installation ensures the provenance of the software and protects against malicious tampering. Muscle Soreness Kindness
NIST SP800-53 SI-7
DISA CCI 352, 663
highnsEnsure Software Patches InstalledIf the system is joined to the Red Hat Network, a Red Hat Satellite Server, or a yum server, run the following command to install updates: # yum update If the system is not configured to use one of these sources, updates (in the form of RPM packages) can be manually downloaded from the Red Hat Network and installed using rpm. Installing software updates is a fundamental mitigation against the exploitation of publicly-known vulnerabilities. Blisters Patience
NIST SP800-53 SI-2
DISA CCI 1227, 1233
Software Integrity Checking Both the AIDE (Advanced Intrusion Detection Environment) software and the RPM package management system provide mechanisms for verifying the integrity of installed software. AIDE is the successor to the well-known Tripwire integrity checker. The RPM package management system can conduct integrity checks by comparing information in its metadata database with files installed on the system. Integrity checking cannot prevent intrusions into your system, but can detect that they have occurred. Requirements for software integrity checking may be highly dependent on the environment in which the system will be used.
Verify Integrity with AIDEAIDE conducts integrity checks by comparing information about files with previously-gathered information. Ideally, the AIDE database should be created immediately after your system is built, and before the system is connected to any network. AIDE is highly configurable, with further configuration information located in /usr/share/doc/aide-VERSION
mediumCCE-27024-9Install AIDE Install the AIDE package with the command: # yum install aide The AIDE package must be installed if it is to be available for integrity checking. Acne Temperance
NIST SP800-53 CM-6(d), CM-6(3), SC-28, SI-7
DISA CCI 1069
lowCCE-TODODisable Prelinking The prelinking feature changes binaries in an attempt to decrease their startup time. In order to disable it, change or add the following line inside the file /etc/sysconfig/prelink: PRELINKING=no Next, the following command to return binaries to a normal, non-prelinked state: # /usr/sbin/prelink -ua The prelinking feature can interfere with the operation of AIDE, because it changes binaries. Blisters Kindness
NIST SP800-53 CM-6(d), CM-6(3), SC-28, SI-7
lownsBuild and Test AIDE DatabaseRun the following command to generate a new database: # /usr/sbin/aide --init By default, the database will be written to the file /var/lib/aide/aide.db.new.gz. Storing the database, the configuration file /etc/aide.conf, and the binary /usr/sbin/aide (or hashes of these files), in a secure location (such as on read-only media) provides additional assurance about their integrity. The newly-generated database can be installed as follows: # cp /var/lib/aide/aide.db.new.gz /var/lib/aide/aide.db.gz To initiate a manual check, run the following command: # /usr/sbin/aide --check If this check produces any unexpected output, investigate. For AIDE to be effective, an initial database of "known-good" information about files must be captured and it should be able to be verified against the installed files. Bruising Charity
NIST SP800-53 CM-6(d), CM-6(3), SC-28, SI-7
mediumCCE-TODOConfigure Periodic Execution of AIDE AIDE should be executed on a periodic basis to check for changes. To implement a daily execution of AIDE at 4:05am using cron, add the following line to /etc/crontab: 05 4 * * * root /usr/sbin/aide --check AIDE can be executed periodically through other means; this is merely one example. By default, AIDE does not install itself for periodic execution. Periodically running AIDE may reveal unexpected changes in installed files. Black Eye Chastity
NIST SP800-53 CM-6(d), CM-6(3), SC-28, SI-7
DISA CCI 374, 416, 1069, 1263, 1297, 1589
Verify Integrity with RPMThe RPM package management system includes the ability to verify the integrity of installed packages by comparing the installed files with information about the files taken from the package metadata stored in the RPM database. Although an attacker could corrupt the RPM database (analogous to attacking the AIDE database as described above), this check can still reveal modification of important files. To list which files on the system differ from what is expected by the RPM database: # rpm -qVa See the man page for rpm to see a complete explanation of each column.
lowCCE-26731-0Verify File Permissions with RPMThe RPM package management system can check file access permissions of installed software packages, including many that are important to system security. The following command will list which files on the system have permissions different from what is expected by the RPM database: # rpm -Va | grep '^.M' Permissions on system binaries and configuration files that are too generous could allow an unauthorized user to gain privileges that they should not have. The permissions set by the vendor should be maintained. Any deviations from this baseline should be investigated. Warts Temperance
NIST SP800-53 AC-6, CM-6(d), CM-6(3)
DISA CCI 1493, 1494, 1495
lowCCE-TODOVerify File Hashes with RPMThe RPM package management system can check the hashes of installed software packages, including many that are important to system security. Run the following command to list which files on the system have hashes that differ from what is expected by the RPM database: # rpm -Va | grep '^..5' A "c" in the second column indicates that a file is a configuration file, which may appropriately be expected to change. If the file that has changed was not expected to then refresh from distribution media or online repositories. rpm -Uvh affected_package OR yum reinstall affected_package The hash on important files like system executables should match the information given by the RPM database. Executables with erroneous hashes could be a sign of nefarious activity on the system. Influenza Kindness
NIST SP800-53 CM-6(d), CM-6(3), SI-7
DISA CCI 1496
Additional Security Software Additional security software that is not provided or supported by Red Hat can be installed to provide complementary or duplicative security capabilities to those provided by the base platform.
highnsInstall Intrusion Detection Software The base Red Hat platform already includes a sophisticated auditing system that can detect intruder activity, as well as SELinux, which provides host-based intrusion prevention capabilities by confining privileged programs and user sessions which may become compromised. Install an additional intrusion detection tool to provide complementary or duplicative monitoring, reporting, and reaction capabilities to those of the base platform. For DoD systems, the McAfee Host Based Security System is provided to fulfill this role. Adding host-based intrusion detection tools can provide the capability to automatically take actions in response to malicious behavior, which can provide additional agility in reacting to network threats. These tools also often include a reporting capability to provide network awareness of system, which may not otherwise exist in an organization's systems management regime. Muscle Cramping Charity
NIST SP800-53 SC-7
DISA CCI 1263
lownsInstall Virus Scanning Software Install virus scanning software, which uses signatures to search for the presence of viruses on the filesystem. The McAfee uvscan virus scanning tool is provided for DoD systems. Ensure virus definition files are no older than 7 days, or their last release. Configure the virus scanning software to perform scans dynamically on all accessed files. If this is not possible, configure the system to scan all altered files on the system on a daily basis. If the system processes inbound SMTP mail, configure the virus scanner to scan all received mail. Virus scanning software can be used to detect if a system has been compromised by computer viruses, as well as to limit their spread to other systems. Overall Wellness Diligence
NIST SP800-53 SC-28, SI-3
DISA CCI 1239, 1668
File Permissions and MasksTraditional Unix security relies heavily on file and directory permissions to prevent unauthorized users from reading or modifying files to which they should not have access. Adhere to the principle of least privilege - configure each file, directory, and filesystem to allow only the access needed in order for that file to serve its purpose. Note: Several of the commands in this section search filesystems for files or directories with certain characteristics, and are intended to be run on every local partition on a given machine. When the variable PART appears in one of the commands below, it means that the command is intended to be run repeatedly, with the name of each local partition substituted for PART in turn. The following command prints a list of ext4 partitions on the local machine, which is the default filesystem for Red Hat Enterprise Linux 6 installations. $ mount -t ext4 | awk '{print $3}' If your site uses a local filesystem type other than ext4 then you will need to modify this command.
Restrict Partition Mount OptionsSystem partitions can be mounted with certain options that limit what files on those partitions can do. These options are set in the /etc/fstab configuration file, and can be used to make certain types of malicious behavior more difficult.
lowCCE-27045-4Add nodev Option to Non-Root Local PartitionsThe nodev mount option prevents files from being interpreted as character or block devices. Legitimate character and block devices should exist only in the /dev directory on the root partition or within chroot jails built for system services. Add the nodev option to the fourth column of /etc/fstab for the line which controls mounting of any non-root local partitions. The nodev mount option prevents files from being interpreted as character or block devices. The only legitimate location for device files is the /dev directory located on the root partition. The only exception to this is chroot jails, for which it is not advised to set nodev on these filesystems. Hives Humility
NIST SP800-53 CM-7
lowCCE-26860-7Add nodev Option to Removable Media PartitionsThe nodev mount option prevents files from being interpreted as character or block devices. Legitimate character and block devices should exist only in the /dev directory on the root partition or within chroot jails built for system services. Add the nodev option to the fourth column of /etc/fstab for the line which controls mounting of any removable media partitions. The only legitimate location for device files is the /dev directory located on the root partition. An exception to this is chroot jails, and it is not advised to set nodev on partitions which contain their root filesystems. Cuts Kindness
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e), CM-7, MP-2
lowCCE-27196-5Add noexec Option to Removable Media PartitionsThe noexec mount option prevents the direct execution of binaries on the mounted filesystem. Users should not be allowed to execute binaries that exist on partitions mounted from removable media (such as a USB key). The noexec option prevents code from being executed directly from the media itself, and may therefore provide a line of defense against certain types of worms or malicious code. Add the noexec option to the fourth column of /etc/fstab for the line which controls mounting of any removable media partitions. Allowing users to execute binaries from removable media such as USB keys exposes the system to potential compromise. Ingrown Toenails Kindness
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e), CM-7, MP-2
DISA CCI 87
lowCCE-27056-1Add nosuid Option to Removable Media PartitionsThe nosuid mount option prevents set-user-identifier (suid) and set-group-identifier (sgid) permissions from taking effect. These permissions allow users to execute binaries with the same permissions as the owner and group of the file respectively. Users should not be allowed to introduce suid and guid files into the system via partitions mounted from removeable media. Add the nosuid option to the fourth column of /etc/fstab for the line which controls mounting of any removable media partitions. The presence of suid and sgid executables should be tightly controlled. Users should not be able to execute suid or sgid binaries from partitions mounted off of removable media. Fever Humility
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e), CM-7, MP-2
lowCCE-26499-4Add nodev Option to /tmp The nodev mount option can be used to prevent device files from being created in /tmp. Legitimate character and block devices should not exist within temporary directories like /tmp. Add the nodev option to the fourth column of /etc/fstab for the line which controls mounting of /tmp. The only legitimate location for device files is the /dev directory located on the root partition. The only exception to this is chroot jails. Cramps Charity
NIST SP800-53 CM-7, MP-2
lowCCE-26720-3Add noexec Option to /tmpThe noexec mount option can be used to prevent binaries from being executed out of /tmp. Add the noexec option to the fourth column of /etc/fstab for the line which controls mounting of /tmp. Allowing users to execute binaries from world-writable directories such as /tmp should never be necessary in normal operation and can expose the system to potential compromise. Spina Bifida Humility
NIST SP800-53 CM-7, MP-2
lowCCE-26762-5Add nosuid Option to /tmpThe nosuid mount option can be used to prevent execution of setuid programs in /tmp. The suid/sgid permissions should not be required in these world-writable directories. Add the nosuid option to the fourth column of /etc/fstab for the line which controls mounting of /tmp. The presence of suid and sgid executables should be tightly controlled. Users should not be able to execute suid or sgid binaries from temporary storage partitions. Bad Breath Humility
NIST SP800-53 CM-7, MP-2
lowCCE-26778-1Add nodev Option to /dev/shmThe nodev mount option can be used to prevent creation of device files in /dev/shm. Legitimate character and block devices should not exist within temporary directories like /dev/shm. Add the nodev option to the fourth column of /etc/fstab for the line which controls mounting of /dev/shm. The only legitimate location for device files is the /dev directory located on the root partition. The only exception to this is chroot jails. Ingrown Toenails Chastity
NIST SP800-53 CM-7, MP-2
lowCCE-26622-1Add noexec Option to /dev/shmThe noexec mount option can be used to prevent binaries from being executed out of /dev/shm. It can be dangerous to allow the execution of binaries from world-writable temporary storage directories such as /dev/shm. Add the noexec option to the fourth column of /etc/fstab for the line which controls mounting of /dev/shm. Allowing users to execute binaries from world-writable directories such as /dev/shm can expose the system to potential compromise. Canker Sores Patience
NIST SP800-53 CM-7, MP-2
lowCCE-26486-1Add nosuid Option to /dev/shmThe nosuid mount option can be used to prevent execution of setuid programs in /dev/shm. The suid/sgid permissions should not be required in these world-writable directories. Add the nosuid option to the fourth column of /etc/fstab for the line which controls mounting of /dev/shm. The presence of suid and sgid executables should be tightly controlled. Users should not be able to execute suid or sgid binaries from temporary storage partitions. Diabetes Charity
NIST SP800-53 CM-7, MP-2
lowCCE-26582-7Bind Mount /var/tmp To /tmpThe /var/tmp directory is a world-writable directory. Bind-mount it to /tmp in order to consolidate temporary storage into one location protected by the same techniques as /tmp. To do so, edit /etc/fstab and add the following line: /tmp /var/tmp none rw,nodev,noexec,nosuid,bind 0 0 See the mount(8) man page for further explanation of bind mounting. Having multiple locations for temporary storage is not required. Unless absolutely necessary to meet requirements, the storage location /var/tmp should be bind mounted to /tmp and thus share the same protections. Arthritis Chastity
NIST SP800-53 CM-7
Restrict Dynamic Mounting and Unmounting of FilesystemsLinux includes a number of facilities for the automated addition and removal of filesystems on a running system. These facilities may increase convenience, but they all bring some risk -- whether direct risk from allowing unprivileged users to introduce arbitrary filesystems to a machine, or risk that software flaws in the automated mount facility itself will allow an attacker to compromise the system. This command can be used to list the types of filesystems that are available to the currently executing kernel: # find /lib/modules/`uname -r`/kernel/fs -type f -name '*.ko' If these filesystems are not required then they should be explicitly disabled in the appropriate /etc/modprobe.d configuration file. Use caution when enabling any such facility, and find out whether better configuration management or user education might solve the same problem with less risk.
lowCCE-27192-4Restrict Console Device Access to Desktop WorkstationsIf the display manager has been altered to allow remote users to log in and the host is configured to run at runlevel 5, change console as well as the xconsole directive in the /etc/security/console.perms to the following: <console>=tty[0-9][0-9]* vc/[0-9][0-9]* :0\.[0-9] :0 <xconsole>=:0\.[0-9] :0When a user logs in, the module pam_console.so called via the command login, or by some of the graphics program of logging, such as gdm, kdm, and xdm. If this user is the first to log into the physical console - called the console user - the user module assures the mastery of a wide variety of devices normally belong to root. Administrative privileges should be limited for non-root users. Review the man page for pam_console for more information Headache Chastity
NIST SP800-53
lowCCE-26892-0Restrict Console Device Access to ServersIf the display manager has been altered to allow remote users to log in and the host is configured to run at runlevel 5, change console as well as the xconsole directive in the /etc/security/console.perms to the following: <console>=tty[0-9][0-9]* vc/[0-9][0-9]*When a user logs in, the module pam_console.so called via the command login, or by some of the graphics program of logging, such as gdm, kdm, and xdm. If this user is the first to log into the physical console - called the console user - the user module assures the mastery of a wide variety of devices normally belong to root. Administrative privileges should be limited for non-root users. Review the man page for pam_console for more information Cold Sore Humility
NIST SP800-53
lowCCE-27016-5Disable Modprobe Loading of USB Storage Driver To prevent USB storage devices from being used, configure the kernel module loading system to prevent automatic loading of the USB storage driver. To configure the system to prevent the usb-storage kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install usb-storage /bin/true This will prevent the modprobe program from loading the usb-storage module, but will not prevent an administrator (or another program) from using the insmod program to load the module manually.USB storage devices such as thumb drives can be used to introduce unauthorized software and other vulnerabilities. Support for these devices should be disabled and the devices themselves should be tightly controlled. Parkinson's Disease Kindness
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e)
DISA CCI 1250, 85
lowCCE-27011-6Disable Kernel Support for USB via Bootloader Configuration Another means of disabling USB storage is to disable all USB support provided by the operating system. This can be accomplished by adding the nousb argument to the kernel's boot loader configuration. To disable kernel support for USB, append "nousb" to the kernel line in /etc/grub.conf as follows: kernel /vmlinuz-VERSION ro vga=ext root=/dev/VolGroup00/LogVol00 rhgb quiet nousb WARNING: Disabling all kernel support for USB will cause problems for systems with USB-based keyboards, mice, or printers. This guidance is inappropriate for systems which require USB connectivity.Disabling the USB subsystem within the Linux kernel at system boot will also disable USB storage devices if they are plugged into the system. Support for these devices should be disabled and the devices themselves should be tightly controlled. Constipation Diligence
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e)
DISA CCI 1250, 85
lowCCE-26952-2Disable Booting from USB DevicesAn attacker with physical access could try to boot the system from a USB flash drive and then access any data on the system's hard drive, circumventing the normal operating system's access controls. To prevent this, configure the BIOS to disallow booting from USB drives. Also configure the BIOS or firmware password as described in the section titled "Set BIOS Password" to prevent unauthorized configuration changes.Booting a system from a USB device would allow an attacker to circumvent any security measures offered by the native OS. Attackers could mount partitions and modify the configuration of the native OS. The BIOS should be configured to disallow booting from USB media. Diarrhea Kindness
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e)
DISA CCI 1250, 85
lowCCE-26976-1Disable the AutomounterThe autofs daemon mounts and unmounts filesystems, such as user home directories shared via NFS, on demand. In addition, autofs can be used to handle removable media, and the default configuration provides the cdrom device as /misc/cd. However, this method of providing access to removable media is not common, so autofs can almost always be disabled if NFS is not in use. Even if NFS is required, it is almost always possible to configure filesystem mounts statically by editing /etc/fstab rather than relying on the automounter. If the autofs service is not needed to dynamically mount NFS filesystems or removable media, disable the service for all runlevels: # chkconfig --level 0123456 autofs off Stop the service if it is already running: # service autofs stop All filesystems that are required for the successful operation of the system should be explicitly listed in /etc/fstab by and administrator. New filesystems should not be arbitrarily introduced via the automounter. Influenza Patience
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e)
DISA CCI 1250, 85
lowCCE-27035-5Disable GNOME AutomountingThe system's default desktop environment, GNOME, will mount devices and removable media (such as DVDs, CDs and USB flash drives) whenever they are inserted into the system. Disable automount and autorun within GNOME by running the following: # gconftool-2 --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type bool \ --set /apps/nautilus/preferences/media_automount false # gconftool-2 --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type bool \ --set /apps/nautilus/preferences/media_autorun_never true These settings can be verified by running the following: $ gconftool-2 --direct \ --config-source xml:read:/etc/gconf/gconf.xml.mandatory \ --get /apps/nautilus/preferences/media_automount $ gconftool-2 --direct \ --config-source xml:read:/etc/gconf/gconf.xml.mandatory \ --get /apps/nautilus/preferences/media_autorun_never The system's capabilities for automatic mounting should be configured to match whatever is defined by security policy. Disabling USB storage as described in the USB section will prevent the use of USB storage devices, but this step should also be taken as an additional layer of protection to prevent automatic mounting of CDs and DVDs. Ingrown Toenails Chastity
NIST SP800-53 AC-19(a), AC-19(d), AC-19(e)
DISA CCI 1250, 85
lowCCE-26340-0Disable Mounting of cramfs To configure the system to prevent the cramfs kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install cramfs /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Halitosis Humility
NIST SP800-53 CM-7
lowCCE-26544-7Disable Mounting of freevxfs To configure the system to prevent the freevsfs kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install freevsfs /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Obesity Kindness
NIST SP800-53 CM-7
lowCCE-26670-0Disable Mounting of jffs2 To configure the system to prevent the jffs2 kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install jffs2 /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Cramps Kindness
NIST SP800-53 CM-7
lowCCE-26800-3Disable Mounting of hfs To configure the system to prevent the hfs kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install hfs /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Overall Wellness Charity
NIST SP800-53 CM-7
lowCCE-26361-6Disable Mounting of hfsplus To configure the system to prevent the hfsplus kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install hfsplus /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Pink Eye Temperance
NIST SP800-53 CM-7
lowCCE-26404-4Disable Mounting of squashfs To configure the system to prevent the squashfs kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install squashfs /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Halitosis Humility
NIST SP800-53 CM-7
lowCCE-26677-5Disable Mounting of udf To configure the system to prevent the udf kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install udf /bin/true This effectively prevents usage of this uncommon filesystem. Linux kernel modules which implement filesystems that are not needed by the local system should be disabled. Sprain Charity
NIST SP800-53 CM-7
lowCCE-TODODisable All GNOME ThumbnailersThe system's default desktop environment, GNOME, uses a number of different thumbnailer programs to generate thumbnails for any new or modified content in an opened folder. The following command can disable the execution of these thumbnail applications: # gconftool-2 --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type bool \ --set /desktop/gnome/thumbnailers/disable_all true This effectively prevents an attacker from gaining access to a system through a flaw in GNOME's Nautilus thumbnail creators. An attacker with knowledge of a flaw in a GNOME thumbnailer application could craft a malicious file to exploit this flaw. Assuming the attacker could place the malicious file on the local filesystem (via a web upload for example) and assuming a user browses the same location using Nautilus, the malicious file would exploit the thumbnailer with the potential for malicious code execution. It is best to disable these thumbnailer applications unless they are explicitly required. Chickenpox Humility
NIST SP800-53 CM-7
Verify Permissions on Important Files and DirectoriesPermissions for many files on a system must be set restrictively to ensure sensitive information is properly protected. This section discusses important permission restrictions which can be verified to ensure that no harmful discrepancies have arisen.
Verify Permissions on Files with Local Account Information and CredentialsThe default restrictive permissions for files which act as important security databases such as passwd, shadow, group, and gshadow files must be maintained. Many utilities need read access to the passwd file in order to function properly, but read access to the shadow file allows malicious attacks against system passwords, and should never be enabled.
mediumCCE-26947-2Verify User Who Owns shadow File To properly set the owner of /etc/shadow, run the command: # chown root /etc/shadow The /etc/shadow file contains the list of local system accounts and stores password hashes. Protection of this file is critical for system security. Failure to give ownership of this file to root provides the designated owner with access to sensitive information which could weaken the system security posture. Diabetes Patience
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26967-0Verify Group Who Owns shadow File To properly set the group owner of /etc/shadow, run the command: # chgrp root /etc/shadow The /etc/shadow file stores password hashes. Protection of this file is critical for system security. Tooth Ache Patience
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26992-8Verify Permissions on shadow File To properly set the permissions of /etc/shadow, run the command: # chmod 0000 /etc/shadow The /etc/shadow file contains the list of local system accounts and stores password hashes. Protection of this file is critical for system security. Failure to give ownership of this file to root provides the designated owner with access to sensitive information which could weaken the system security posture. Pain Humility
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26822-7Verify User Who Owns group File To properly set the owner of /etc/group, run the command: # chown root /etc/group The /etc/group file contains information regarding groups that are configured on the system. Protection of this file is important for system security. Halitosis Humility
NIST SP800-53 AC-6
mediumCCE-26930-8Verify Group Who Owns group File To properly set the group owner of /etc/group, run the command: # chgrp root /etc/group The /etc/group file contains information regarding groups that are configured on the system. Protection of this file is important for system security. Fever Kindness
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26954-8Verify Permissions on group File To properly set the permissions of /etc/group, run the command: # chmod 644 /etc/group The /etc/group file contains information regarding groups that are configured on the system. Protection of this file is important for system security. Pink Eye Humility
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-27026-4Verify User Who Owns gshadow File To properly set the owner of /etc/gshadow, run the command: # chown root /etc/gshadow The /etc/gshadow file contains group password hashes. Protection of this file is critical for system security. High Cholesterol Temperance
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26975-3Verify Group Who Owns gshadow File To properly set the group owner of /etc/gshadow, run the command: # chgrp root /etc/gshadow The /etc/gshadow file contains group password hashes. Protection of this file is critical for system security. Tooth Ache Humility
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26951-4Verify Permissions on gshadow File To properly set the permissions of /etc/gshadow, run the command: # chmod 0000 /etc/gshadow The /etc/gshadow file contains group password hashes. Protection of this file is critical for system security. Bloody Nose Humility
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26953-0Verify User Who Owns passwd File To properly set the owner of /etc/passwd, run the command: # chown root /etc/passwd The /etc/passwd file contains information about the users that are configured on the system. Protection of this file is critical for system security. Overall Wellness Diligence
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26856-5Verify Group Who Owns passwd File To properly set the group owner of /etc/passwd, run the command: # chgrp root /etc/passwd The /etc/passwd file contains information about the users that are configured on the system. Protection of this file is critical for system security. Muscle Cramping Temperance
NIST SP800-53 AC-6
DISA CCI 225
mediumCCE-26868-0Verify Permissions on passwd File To properly set the permissions of /etc/passwd, run the command: # chmod 0644 /etc/passwd If the /etc/passwd file is writable by a group-owner or the world the risk of its compromise is increased. The file contains the list of accounts on the system and associated information, and protection of this file is critical for system security. Sore Throat Charity
NIST SP800-53 AC-6
DISA CCI 225
Verify File Permissions Within Some Important DirectoriesSome directories contain files whose confidentiality or integrity is notably important and may also be susceptible to misconfiguration over time, particularly if unpackaged software is installed. As such, an argument exists to verify that files' permissions within these directories remain configured correctly and restrictively.
mediumnsVerify that Shared Library Files Have Restrictive PermissionsSystem-wide shared library files, which are linked to executables during process load time or run time, are stored in the following directories by default: /lib /lib64 /usr/lib /usr/lib64 Kernel modules, which can be added to the kernel during runtime, are stored in /lib/modules. All files in these directories should not be group-writable or world-writable. If any file in these directories is found to be group-writable or world-writable, correct its permission with the following command: # chmod go-w FILE Files from shared library directories are loaded into the address space of processes (including privileged ones) or of the kernel itself at runtime. Restrictive permissions are necessary to protect the integrity of the system. Hangover Temperance
NIST SP800-53 AC-6
DISA CCI 1499
mediumnsVerify that Shared Library Files Have Root OwnershipSystem-wide shared library files, which are linked to executables during process load time or run time, are stored in the following directories by default: /lib /lib64 /usr/lib /usr/lib64 Kernel modules, which can be added to the kernel during runtime, are also stored in /lib/modules. All files in these directories should be owned by the root user. If any file in these directories is found to be owned by a user other than root, correct its ownership with the following command: # chown root FILE Files from shared library directories are loaded into the address space of processes (including privileged ones) or of the kernel itself at runtime. Proper ownership is necessary to protect the integrity of the system. Canker Sores Temperance
NIST SP800-53 AC-6
DISA CCI 1499
mediumnsVerify that System Executables Have Restrictive Permissions System executables are stored in the following directories by default: /bin /usr/bin /usr/local/bin /sbin /usr/sbin /usr/local/sbin All files in these directories should not be group-writable or world-writable. If any file FILE in these directories is found to be group-writable or world-writable, correct its permission with the following command: # chmod go-w FILE System binaries are executed by privileged users, as well as system services, and restrictive permissions are necessary to ensure execution of these programs cannot be co-opted. Jaundice Charity
NIST SP800-53 AC-6
DISA CCI 1499
mediumnsVerify that System Executables Have Root Ownership System executables are stored in the following directories by default: /bin /usr/bin /usr/local/bin /sbin /usr/sbin /usr/local/sbin All files in these directories should be owned by the root user. If any file FILE in these directories is found to be owned by a user other than root, correct its ownership with the following command: # chown root FILE System binaries are executed by privileged users as well as system services, and restrictive permissions are necessary to ensure that their execution of these programs cannot be co-opted. Burns Humility
NIST SP800-53 AC-6
DISA CCI 1499
lowCCE-26840-9Verify that All World-Writable Directories Have Sticky Bits SetWhen the so-called 'sticky bit' is set on a directory, only the owner of a given file may remove that file from the directory. Without the sticky bit, any user with write access to a directory may remove any file in the directory. Setting the sticky bit prevents users from removing each other's files. In cases where there is no reason for a directory to be world-writable, a better solution is to remove that permission rather than to set the sticky bit. However, if a directory is used by a particular application, consult that application's documentation instead of blindly changing modes. To set the sticky bit on a world-writable directory DIR, run the following command: # chmod +t DIR Failing to set the sticky bit on public directories allows unauthorized users to delete files in the directory structure. The only authorized public directories are those temporary directories supplied with the system, or those designed to be temporary file repositories. The setting is normally reserved for directories used by the system, by users for temporary file storage (such as /tmp), and for directories requiring global read/write access. Bedwetting Chastity
NIST SP800-53 AC-6
mediumCCE-26910-0Ensure No World-Writable Files ExistIt is generally a good idea to remove global (other) write access to a file when it is discovered. However, check with documentation for specific applications before making changes. Also, monitor for recurring world-writable files, as these may be symptoms of a misconfigured application or user account. Data in world-writable files can be modified by any user on the system. In almost all circumstances, files can be configured using a combination of user and group permissions to support whatever legitimate access is needed without the risk caused by world-writable files. Hives Kindness
NIST SP800-53 AC-6
lowCCE-26769-0Ensure All Setgid Executables Are AuthorizedThe SGID (set group id) bit should be set only on files that were installed via authorized means. A straightforward means of identifying unauthorized SGID files is determine if any were not installed as part of an RPM package, which is cryptographically verified. Investigate the origin of any unpackaged SGID files. Executable files with the SGID permission run with the privileges of the owner of the file. SGID files of uncertain provenance could allow for unprivileged users to elevate privileges. The presence of these files should be strictly controlled on the system. Upset Stomach Humility
NIST SP800-53
lowCCE-26497-8Ensure All SUID Executables Are AuthorizedThe SUID (set user id) bit should be set only on files that were installed via authorized means. A straightforward means of identifying unauthorized SGID files is determine if any were not installed as part of an RPM package, which is cryptographically verified. Investigate the origin of any unpackaged SUID files. Executable files with the SUID permission run with the privileges of the owner of the file. SUID files of uncertain provenance could allow for unprivileged users to elevate privileges. The presence of these files should be strictly controlled on the system. Pink Eye Charity
NIST SP800-53
lowCCE-27032-2Ensure All Files Are Owned by a UserIf any files are not owned by a user, then the cause of their lack of ownership should be investigated. Following this, the files should be deleted or assigned to an appropriate user. Unowned files do not directly imply a security problem, but they are generally a sign that something is amiss. They may be caused by an intruder, by incorrect software installation or draft software removal, or by failure to remove all files belonging to a deleted account. The files should be repaired so they will not cause problems when accounts are created in the future, and the cause should be discovered and addressed. Red Eyes Charity
NIST SP800-53 AC-6
DISA CCI 224
lowCCE-26872-2Ensure All Files Are Owned by a GroupIf any files are not owned by a group, then the cause of their lack of group-ownership should be investigated. Following this, the files should be deleted or assigned to an appropriate group. Unowned files do not directly imply a security problem, but they are generally a sign that something is amiss. They may be caused by an intruder, by incorrect software installation or draft software removal, or by failure to remove all files belonging to a deleted account. The files should be repaired so they will not cause problems when accounts are created in the future, and the cause should be discovered and addressed. Cuts Temperance
NIST SP800-53 AC-6
DISA CCI 224
lowCCE-26642-9Ensure All World-Writable Directories Are Owned by a System AccountAll directories in local partitions which are world-writable should be owned by root or another system account. If any world-writable directories are not owned by a system account, this should be investigated. Following this, the files should be deleted or assigned to an appropriate group. Allowing a user account to own a world-writable directory is undesirable because it allows the owner of that directory to remove or replace any files that may be placed in the directory by other users. Motion Sickness Temperance
NIST SP800-53 AC-6
Restrict Programs from Dangerous Execution PatternsThe recommendations in this section are designed to ensure that the system's features to protect against potentially dangerous program execution are activated. These protections are applied at the system initialization or kernel level, and defend against certain types of badly-configured or compromised programs.
Daemon UmaskThe umask is a per-process setting which limits the default permissions for creation of new files and directories. The system includes initialization scripts which set the default umask for system daemons.
lowCCE-27031-4Set Daemon UmaskThe file /etc/init.d/functions includes initialization parameters for most or all daemons started at boot time. The default umask of 022 prevents creation of group- or world-writable files. To set the default umask for daemons, edit the following line, inserting 022 or 027 for UMASK appropriately: umask UMASK Setting the umask to too restrictive a setting can cause serious errors at runtime. Many daemons on the system already individually restrict themselves to a umask of 077 in their own init scripts. The umask influences the permissions assigned to files created by a process at run time. An unnecessarily permissive umask could result in files being created with insecure permissions. Bloody Nose Kindness
NIST SP800-53 AC-6
Disable Core DumpsA core dump file is the memory image of an executable program when it was terminated by the operating system due to errant behavior. In most cases, only software developers legitimately need to access these files. The core dump files may also contain sensitive information, or unnecessarily occupy large amounts of disk space. Once a hard limit is set in /etc/security/limits.conf, a user cannot increase that limit within his or her own session. If access to core dumps is required, consider restricting them to only certain users or groups. See the limits.conf man page for more information. The core dumps of setuid programs are further protected. The sysctl variable fs.suid_dumpable controls whether the kernel allows core dumps from these programs at all. The default value of 0 is recommended.
lowCCE-27033-0Disable Core Dumps for All UsersTo disable core dumps for all users, add the following line to /etc/security/limits.conf: * hard core 0 A core dump includes a memory image taken at the time the operating system terminates an application. The memory image could contain sensitive data and is generally useful only for developers trying to debug problems. Black Eye Kindness
NIST SP800-53 SC-5
lowCCE-27044-7Disable Core Dumps for SUID programs To set the runtime status of the fs.suid_dumpable kernel parameter, run the following command: # sysctl -w fs.suid_dumpable=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: fs.suid_dumpable = 0 The core dump of a setuid program is more likely to contain sensitive data, as the program itself runs with greater privileges than the user who initiated execution of the program. Disabling the ability for any setuid program to write a core file decreases the risk of unauthorized access of such data. Bruising Kindness
NIST SP800-53 SI-11
Enable ExecShieldExecShield describes kernel features that provide protection against exploitation of memory corruption errors such as buffer overflows. These features include random placement of the stack and other memory regions, prevention of execution in memory that should only hold data, and special handling of text buffers. These protections are enabled by default and controlled through sysctl variables kernel.exec-shield and kernel.randomize_va_space.
mediumCCE-27007-4Enable ExecShield To set the runtime status of the kernel.exec-shield kernel parameter, run the following command: # sysctl -w kernel.exec-shield=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: kernel.exec-shield = 1 ExecShield uses the segmentation feature on all x86 systems to prevent execution in memory higher than a certain address. It writes an address as a limit in the code segment descriptor, to control where code can be executed, on a per-process basis. When the kernel places a process's memory regions such as the stack and heap higher than this address, the hardware prevents execution in that address range. Bad Breath Humility
NIST SP800-53
mediumCCE-26999-3Enable Randomized Layout of Virtual Address Space To set the runtime status of the kernel.randomize_va_space kernel parameter, run the following command: # sysctl -w kernel.randomize_va_space=2 If this is not the system's default value, add the following line to /etc/sysctl.conf: kernel.randomize_va_space = 2 Address space layout randomization (ASLR) makes it more difficult for an attacker to predict the location of attack code they have introduced into a process's address space during an attempt at exploitation. Additionally, ASLR makes it more difficult for an attacker to know the location of existing code in order to re-purpose it using return oriented programming (ROP) techniques. Arthritis Diligence
NIST SP800-53
Enable Execute Disable (XD) or No Execute (NX) Support on x86 SystemsRecent processors in the x86 family support the ability to prevent code execution on a per memory page basis. Generically and on AMD processors, this ability is called No Execute (NX), while on Intel processors it is called Execute Disable (XD). This ability can help prevent exploitation of buffer overflow vulnerabilities and should be activated whenever possible. Extra steps must be taken to ensure that this protection is enabled, particularly on 32-bit x86 systems. Other processors, such as Itanium and POWER, have included such support since inception and the standard kernel for those platforms supports the feature.
lowCCE-27010-8Install PAE Kernel on Supported 32-bit x86 SystemsSystems that are using the 64-bit x86 kernel package do not need to install the kernel-PAE package because the 64-bit x86 kernel already includes this support. However, if the system is 32-bit and also supports the PAE and NX features as determined in the previous section, the kernel-PAE package should be installed to enable XD or NX support: # yum install kernel-PAE The installation process should also have configured the bootloader to load the new kernel at boot. Verify this at reboot and modify /etc/grub.conf if necessary.On 32-bit systems that support the XD or NX bit, the vendor-supplied PAE kernel is required to enable either Execute Disable (XD) or No Execute (NX) support. Burns Temperance
NIST SP800-53
lowCCE-27012-4Enable NX or XD Support in the BIOSReboot the system and enter the BIOS or Setup configuration menu. Navigate the BIOS configuration menu and make sure that the option is enabled. The setting may be located under a Security section. Look for Execute Disable (XD) on Intel-based systems and No Execute (NX) on AMD-based systems.Computers with the ability to prevent this type of code execution frequently put an option in the BIOS that will allow users to turn the feature on or off at will. Snake Bite Kindness
NIST SP800-53
SELinuxSELinux is a feature of the Linux kernel which can be used to guard against misconfigured or compromised programs. SELinux enforces the idea that programs should be limited in what files they can access and what actions they can take. The default SELinux policy, as configured on RHEL6, has been sufficiently developed and debugged that it should be usable on almost any Red Hat machine with minimal configuration and a small amount of system administrator training. This policy prevents system services - including most of the common network-visible services such as mail servers, ftp servers, and DNS servers - from accessing files which those services have no valid reason to access. This action alone prevents a huge amount of possible damage from network attacks against services, from trojaned software, and so forth. This guide recommends that SELinux be enabled using the default (targeted) policy on every Red Hat system, unless that system has requirements which make a stronger policy appropriate.
Enable SELinuxEdit the file /etc/selinux/config. Add or correct the following lines: SELINUX=enforcing SELINUXTYPE=targeted Edit the file /etc/grub.conf. Ensure that the following arguments DO NOT appear on any kernel command line in the file: selinux=0 enforcing=0 The directive SELINUX=enforcing enables SELinux at boot time. If SELinux is suspected of involvement with boot-time problems (unlikely), it is possible to boot into the warning-only mode SELINUX=permissive for debugging purposes. Make certain to change the mode back to enforcing after debugging, set the filesystems to be relabeled for consistency using the command touch /.autorelabel, and reboot. However, the RHEL6 default SELinux configuration should be sufficiently reasonable that most systems will boot without serious problems. Some applications that require deep or unusual system privileges, such as virtual machine software, may not be compatible with SELinux in its default configuration. However, this should be uncommon, and SELinux's application support continues to improve. In other cases, SELinux may reveal unusual or insecure program behavior by design. The directive SELINUXTYPE=targeted configures SELinux to use the default targeted policy. The SELinux boot mode specified in /etc/selinux/config can be overridden by command-line arguments passed to the kernel. It is necessary to check grub.conf to ensure that this has not been done and to protect the boot process.
mediumCCE-26956-3Ensure SELinux Not Disabled in /etc/grub.confSELinux can be disabled at boot time by an argument in /etc/grub.conf. Remove any instances of selinux=0 from the kernel arguments in that file to prevent SELinux from being disabled at boot. Disabling a major host protection feature, such as SELinux, at boot time prevents it from confining system services at boot time. Further, it increases the chances that it will remain off during system operation. Pain Humility
NIST SP800-53 AC-3, AC-6, AU-9
DISA CCI 22, 32
mediumCCE-26969-6Ensure SELinux State is EnforcingThe SELinux state should be set to enforcing at system boot time. In the file /etc/selinux/config, add or correct the following line to configure the system to boot into enforcing mode: SELINUX=enforcing Setting the SELinux state to enforcing ensures SELinux is able to confine potentially compromised processes to the security policy, which is designed to prevent them from causing damage to the system or further elevating their privileges. Overall Wellness Kindness
NIST SP800-53 AC-3, AC-4, AC-6, AU-9
DISA CCI 22, 32, 26
lowCCE-26875-5Configure SELinux PolicyThe SELinux targeted policy is appropriate for general-purpose desktops and servers, as well as systems in many other roles. To configure the system to use this policy, add or correct the following line in /etc/selinux/config: SELINUXTYPE=targeted Other policies, such as mls, provide additional security labeling and greater confinement but are not compatible with many general-purpose use cases. Setting the SELinux policy to targeted or a more specialized policy ensures the system will confine processes that are likely to be targeted for exploitation, such as network or system services. Bedwetting Chastity
NIST SP800-53 AC-3, AC-4, AC-6, AU-9
DISA CCI 22, 32
lowCCE-26991-0Enable the SELinux Context Restoration Service (restorecond)The restorecond service utilizes inotify to look for the creation of new files listed in the /etc/selinux/restorecond.conf configuration file. When a file is created, restorecond ensures the file receives the proper SELinux security context. The restorecond service can be enabled with the following command: # chkconfig restorecond on The restorecond service helps ensure that the default SELinux file context is applied to files. This allows automatic correction of file contexts created by some programs. Ingrown Toenails Chastity
NIST SP800-53 AC-3, AC-4, AC-6, AU-9
mediumnsEnsure No Daemons are Unconfined by SELinux Daemons for which the SELinux policy does not contain rules will inherit the context of the parent process. Because daemons are launched during startup and descend from the init process, they inherit the initrc_t context. To check for unconfined daemons, run the following command: # ps -eZ | egrep "initrc" | egrep -vw "tr|ps|egrep|bash|awk" | tr ':' ' ' | awk '{ print $NF }' It should produce no output in a well-configured system. Daemons which run with the initrc_t context may cause AVC denials, or allow privileges that the daemon does not require. Bad Breath Temperance
NIST SP800-53 AC-6, AU-9, CM-7
lowCCE-26774-0Ensure No Device Files are Unlabeled by SELinuxDevice files, which are used for communication with important system resources, should be labeled with proper SELinux types. If any device files carry the SELinux type unlabeled_t, investigate the cause and correct the file's context. If a device file carries the SELinux type unlabeled_t, then SELinux cannot properly restrict access to the device file. Headache Kindness
NIST SP800-53 AC-6, AU-9, CM-7
DISA CCI 22, 32
Account and Access ControlIn traditional Unix security, if an attacker gains shell access to a certain login account, they can perform any action or access any file to which that account has access. Therefore, making it more difficult for unauthorized people to gain shell access to accounts, particularly to privileged accounts, is a necessary part of securing a system. This section introduces mechanisms for restricting access to accounts under RHEL6.
Protect Accounts by Restricting Password-Based LoginConventionally, Unix shell accounts are accessed by providing a username and password to a login program, which tests these values for correctness using the /etc/passwd and /etc/shadow files. Password-based login is vulnerable to guessing of weak passwords, and to sniffing and man-in-the-middle attacks against passwords entered over a network or at an insecure console. Therefore, mechanisms for accessing accounts by entering usernames and passwords should be restricted to those which are operationally necessary.
Restrict Root Logins Direct root logins should be allowed only for emergency use. In normal situations, the administrator should access the system via a unique unprivileged account, and then use su or sudo to execute privileged commands. Discouraging administrators from accessing the root account directly ensures an audit trail in organizations with multiple administrators. Locking down the channels through which root can connect directly also reduces opportunities for password-guessing against the root account. The login program uses the file /etc/securetty to determine which interfaces should allow root logins. The virtual devices /dev/console and /dev/tty* represent the system consoles (accessible via the Ctrl-Alt-F1 through Ctrl-Alt-F6 keyboard sequences on a default installation). The default securetty file also contains /dev/vc/*. These are likely to be deprecated in most environments, but may be retained for compatibility. Root should also be prohibited from connecting via network protocols. Other sections of this document include guidance describing how to prevent root from logging in via SSH.
mediumCCE-26855-7Restrict Virtual Console Root Logins To restrict root logins through the (deprecated) virtual console devices, ensure lines of this form do not appear in /etc/securetty: vc/1 vc/2 vc/3 vc/4 Preventing direct root login to virtual console devices helps ensure accountability for actions taken on the system using the root account. Migraine Kindness
NIST SP800-53 AC-6(2)
DISA CCI 770
lowCCE-27047-0Restrict Serial Port Root LoginsTo restrict root logins on serial ports, ensure lines of this form do not appear in /etc/securetty: ttyS0 ttyS1 Preventing direct root login to serial port interfaces helps ensure accountability for actions taken on the systems using the root account. Asthma Temperance
NIST SP800-53 AC-6(2)
DISA CCI 770
lownsRestrict Web Browser Use for Administrative Accounts Enforce policy requiring administrative accounts use web browsers only for local service administration. If a browser vulnerability is exploited while running with administrative privileges, the entire system could be compromised. Specific exceptions for local service administration should be documented in site-defined policy. Warts Temperance
mediumCCE-26966-2Ensure that System Accounts Do Not Run a Shell Upon Login Some accounts are not associated with a human user of the system, and exist to perform some administrative function. Should an attacker be able to log into these accounts, they should not be granted access to a shell. The login shell for each local account is stored in the last field of each line in /etc/passwd. System accounts are those user accounts with a user ID less than 500. The user ID is stored in the third field. If any system account SYSACCT (other than root) has a login shell, disable it with the command: # usermod -s /sbin/nologin SYSACCT Ensuring shells are not given to system accounts upon login makes it more difficult for attackers to make use of system accounts. Diabetes Chastity
NIST SP800-53
mediumCCE-26971-2Verify Only Root Has UID 0 If any account other than root has a UID of 0, this misconfiguration should be investigated and the accounts other than root should be removed or have their UID changed. An account has root authority if it has a UID of 0. Multiple accounts with a UID of 0 afford more opportunity for potential intruders to guess a password for a privileged account. Proper configuration of sudo is recommended to afford multiple system administrators access to root privileges in an accountable manner. Warts Chastity
NIST SP800-53 AC-6, AC-11
DISA CCI 366
lownsRoot Path Must Be Vendor Default Assuming root shell is bash, edit the following files: ~/.profile ~/.bashrc Change any PATH variables to the vendor default for root and remove any empty PATH entries or references to relative paths. The root account's executable search path must be the vendor default, and must contain only absolute paths. Sty Chastity
Verify Proper Storage and Existence of Password Hashes By default, password hashes for local accounts are stored in the second field (colon-separated) in /etc/shadow. This file should be readable only by processes running with root credentials, preventing users from casually accessing others' password hashes and attempting to crack them. However, it remains possible to misconfigure the system and store password hashes in world-readable files such as /etc/passwd, or to even store passwords themselves in plaintext on the system. Using system-provided tools for password change/creation should allow administrators to avoid such misconfiguration.
highCCE-27038-9Prevent Log In to Accounts With Empty PasswordIf an account is configured for password authentication but does not have an assigned password, it may be possible to log into the account without authentication. Remove any instances of the nullok option in /etc/pam.d/system-auth to prevent logins with empty passwords. If an account has an empty password, anyone could log in and run commands with the privileges of that account. Accounts with empty passwords should never be used in operational environments. Anemia Humility
NIST SP800-53 IA-5
mediumCCE-26476-2Verify All Account Password Hashes are Shadowed If any password hashes are stored in /etc/passwd (in the second field, instead of an x), the cause of this misconfiguration should be investigated. The account should have its password reset and the hash should be properly stored, or the account should be deleted entirely. The hashes for all user account passwords should be stored in the file /etc/shadow and never in /etc/passwd, which is readable by all users. Motion Sickness Temperance
NIST SP800-53 IA-5
DISA CCI 201
lownsAll GIDs referenced in /etc/passwd must be defined in /etc/group Add a group to the system for each GID referenced without a corresponding group. Inconsistency in GIDs between /etc/passwd and /etc/group could lead to a user having unintended rights. Bruising Charity
DISA CCI 366
mediumCCE-TODOVerify No netrc Files ExistThe .netrc files contain login information used to auto-login into FTP servers and reside in the user's home directory. These files may contain unencrypted passwords to remote FTP servers making them susceptible to access by unauthorized users and should not be used. Any .netrc files should be removed. Unencrypted passwords for remote FTP servers may be stored in .netrc files. DoD policy requires passwords be encrypted in storage and not used in access scripts. Gallstones Kindness
NIST SP800-53 IA-5
DISA CCI 196
Set Password Expiration ParametersThe file /etc/login.defs controls several password-related settings. Programs such as passwd, su, and login consult /etc/login.defs to determine behavior with regard to password aging, expiration warnings, and length. See the man page login.defs(5) for more information. Users should be forced to change their passwords, in order to decrease the utility of compromised passwords. However, the need to change passwords often should be balanced against the risk that users will reuse or write down passwords if forced to change them too often. Forcing password changes every 90-360 days, depending on the environment, is recommended. Set the appropriate value as PASS_MAX_DAYS and apply it to existing accounts with the -M flag. The PASS_MIN_DAYS (-m) setting prevents password changes for 7 days after the first change, to discourage password cycling. If you use this setting, train users to contact an administrator for an emergency password change in case a new password becomes compromised. The PASS_WARN_AGE (-W) setting gives users 7 days of warnings at login time that their passwords are about to expire. For example, for each existing human user USER, expiration parameters could be adjusted to a 180 day maximum password age, 7 day minimum password age, and 7 day warning period with the following command: # chage -M 180 -m 7 -W 7 USER
mediumCCE-27002-5Set Password Minimum Length in login.defsTo specify password length requirements for new accounts, edit the file /etc/login.defs and add or correct the following lines: PASS_MIN_LEN 14 The DoD requirement is 14. If a program consults /etc/login.defs and also another PAM module (such as pam_cracklib) during a password change operation, then the most restrictive must be satisfied. See PAM section for more information about enforcing password quality requirements. Requiring a minimum password length makes password cracking attacks more difficult by ensuring a larger search space. However, any security benefit from an onerous requirement must be carefully weighed against usability problems, support costs, or counterproductive behavior that may result. Diaper Rash Diligence
NIST SP800-53 IA-5
DISA CCI 205
mediumCCE-27013-2Set Password Minimum AgeTo specify password minimum age for new accounts, edit the file /etc/login.defs and add or correct the following line, replacing DAYS appropriately: PASS_MIN_DAYS DAYS A value of 1 day is considered for sufficient for many environments. The DoD requirement is 1. Setting the minimum password age protects against users cycling back to a favorite password after satisfying the password reuse requirement. Migraine Diligence
NIST SP800-53 IA-5
DISA CCI 198
mediumCCE-26985-2Set Password Maximum AgeTo specify password maximum age for new accounts, edit the file /etc/login.defs and add or correct the following line, replacing DAYS appropriately: PASS_MAX_DAYS DAYS A value of 180 days is sufficient for many environments. The DoD requirement is 60. Setting the password maximum age ensures users are required to periodically change their passwords. This could possibly decrease the utility of a stolen password. Requiring shorter password lifetimes increases the risk of users writing down the password in a convenient location subject to physical compromise. Pink Eye Chastity
NIST SP800-53 IA-5
DISA CCI 180, 199
lowCCE-26988-6Set Password Warning AgeTo specify how many days prior to password expiration that a warning will be issued to users, edit the file /etc/login.defs and add or correct the following line, replacing DAYS appropriately: PASS_WARN_AGE DAYS The DoD requirement is 7. Setting the password warning age enables users to make the change at a practical time. Ingrown Toenails Kindness
NIST SP800-53 IA-5
Set Account Expiration ParametersAccounts can be configured to be automatically disabled after a certain time period, meaning that they will require administrator interaction to become usable again. Expiration of accounts after inactivity can be set for all accounts by default and also on a per-account basis, such as for accounts that are known to be temporary. To configure automatic expiration of an account following the expiration of its password (that is, after the password has expired and not been changed), run the following command, substituting NUM_DAYS and USER appropriately: # chage -I NUM_DAYS USER Accounts, such as temporary accounts, can also be configured to expire on an explicitly-set date with the -E option. The file /etc/default/useradd controls default settings for all newly-created accounts created with the system's normal command line utilities.
lownsSet Account Expiration Following InactivityTo specify the number of days after a password expires (which signifies inactivity) until an account is permanently disabled, add or correct the following lines in /etc/default/useradd, substituting NUM_DAYS appropriately: INACTIVE=NUM_DAYS A value of 35 is recommended. If a password is currently on the verge of expiration, then 35 days remain until the account is automatically disabled. However, if the password will not expire for another 60 days, then 95 days could elapse until the account would be automatically disabled. See the useradd man page for more information. Determining the inactivity timeout must be done with careful consideration of the length of a "normal" period of inactivity for users in the particular environment. Setting the timeout too low incurs support costs and also has the potential to impact availability of the system to legitimate users. Disabling inactive accounts ensures that accounts which may not have been responsibly removed are not available to attackers who may have compromised their credentials. Dandruff Humility
NIST SP800-53 AC-2(2), AC-2(3)
DISA CCI 16, 17, 795
lownsEnsure All Accounts on the System Have Unique Names Change usernames, or delete accounts, so each has a unique name. Unique usernames allow for accountability on the system. Migraine Humility
DISA CCI 770, 804
lownsAssign Expiration Date to Temporary Accounts In the event temporary or emergency accounts are required, configure the system to terminate them after a documented time period. For every temporary and emergency account, run the following command to set an expiration date on it, substituting USER and YYYY-MM-DD appropriately: # chage -E YYYY-MM-DD USER YYYY-MM-DD indicates the documented expiration date for the account. When temporary and emergency accounts are created, there is a risk they may remain in place and active after the need for them no longer exists. Account expiration greatly reduces the risk of accounts being misused or hijacked. Red Eyes Patience
NIST SP800-53 AC-2(2), AC-2(3)
DISA CCI 16, 1682
Protect Accounts by Configuring PAMPAM, or Pluggable Authentication Modules, is a system which implements modular authentication for Linux programs. PAM provides a flexible and configurable architecture for authentication, and it should be configured to minimize exposure to unnecessary risk. This section contains guidance on how to accomplish that. PAM is implemented as a set of shared objects which are loaded and invoked whenever an application wishes to authenticate a user. Typically, the application must be running as root in order to take advantage of PAM, because PAM's modules often need to be able to access sensitive stores of account information, such as /etc/shadow. Traditional privileged network listeners (e.g. sshd) or SUID programs (e.g. sudo) already meet this requirement. An SUID root application, userhelper, is provided so that programs which are not SUID or privileged themselves can still take advantage of PAM. PAM looks in the directory /etc/pam.d for application-specific configuration information. For instance, if the program login attempts to authenticate a user, then PAM's libraries follow the instructions in the file /etc/pam.d/login to determine what actions should be taken. One very important file in /etc/pam.d is /etc/pam.d/system-auth. This file, which is included by many other PAM configuration files, defines 'default' system authentication measures. Modifying this file is a good way to make far-reaching authentication changes, for instance when implementing a centralized authentication service.
Set Password Quality RequirementsThe default pam_cracklib PAM module provides strength checking for passwords. It performs a number of checks, such as making sure passwords are not similar to dictionary words, are of at least a certain length, are not the previous password reversed, and are not simply a change of case from the previous password. It can also require passwords to be in certain character classes. The pam_passwdqc PAM module also provides the ability to enforce stringent password strength requirements. It is provided in an RPM of the same name. The man pages pam_cracklib(8) and pam_passwdqc(8) provide information on the capabilities and configuration of each.
Set Password Quality Requirements, if using pam_cracklibThe pam_cracklib PAM module can be configured to meet requirements for a variety of policies. For example, to configure pam_cracklib to require at least one uppercase character, lowercase character, digit, and other (special) character, locate the following line in /etc/pam.d/system-auth: password requisite pam_cracklib.so try_first_pass retry=3 and then alter it to read: password required pam_cracklib.so try_first_pass retry=3 minlen=14 dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 The arguments can be modified to ensure compliance with your organization's security policy. Discussion of each parameter follows.
lowCCE-26796-5Set Password Retry Prompts Permitted Per-SessionTo configure the number of retry prompts that are permitted per-session: Edit the pam_cracklib.so statement in /etc/pam.d/system-auth to show retry=3. The DoD requirement is 3 prompts per session. Setting the password retry prompts that are permitted on a per-session basis to a low value requires some software, such as SSH, to re-connect. This can slow down and draw additional attention to some types of password-guessing attacks. Note that this is different from account lockout, which is provided by the pam_faillock module. Alzheimer’s Disease Kindness
NIST SP800-53 IA-5
DISA CCI 1092
lowCCE-TODOSet Password to Maximum of Three Consecutive Repeating CharactersThe pam_cracklib module's maxrepeat parameter controls requirements for consecutive repeating characters. Edit the /etc/pam.d/system-auth file to include the following line prior to the password include system-auth line: password required pam_cracklib.so maxrepeat=3 Passwords with excessive repeating characters may be more vulnerable to password-guessing attacks. Stinging Nettle Humility
DISA CCI 366
lowCCE-26374-9Set Password Strength Minimum Digit CharactersThe pam_cracklib module's dcredit parameter controls requirements for usage of digits in a password. When set to a negative number, any password will be required to contain that many digits. When set to a positive number, pam_cracklib will grant +1 additional length credit for each digit. Add dcredit=-1 after pam_cracklib.so to require use of a digit in passwords. Requiring digits makes password guessing attacks more difficult by ensuring a larger search space. Sty Kindness
NIST SP800-53 IA-5, 194
DISA CCI 194
lowCCE-26601-5Set Password Strength Minimum Uppercase CharactersThe pam_cracklib module's ucredit= parameter controls requirements for usage of uppercase letters in a password. When set to a negative number, any password will be required to contain that many uppercase characters. When set to a positive number, pam_cracklib will grant +1 additional length credit for each uppercase character. Add ucredit=-1 after pam_cracklib.so to require use of an upper case character in passwords. Requiring a minimum number of uppercase characters makes password guessing attacks more difficult by ensuring a larger search space. Upset Stomach Kindness
NIST SP800-53 IA-5
DISA CCI 192
lowCCE-26409-3Set Password Strength Minimum Special CharactersThe pam_cracklib module's ocredit= parameter controls requirements for usage of special (or ``other'') characters in a password. When set to a negative number, any password will be required to contain that many special characters. When set to a positive number, pam_cracklib will grant +1 additional length credit for each special character. Add ocredit=-1 after pam_cracklib.so to require use of a special character in passwords. Requiring a minimum number of special characters makes password guessing attacks more difficult by ensuring a larger search space. Parkinson's Disease Humility
NIST SP800-53 IA-5
DISA CCI 1619
lowCCE-26631-2Set Password Strength Minimum Lowercase CharactersThe pam_cracklib module's lcredit= parameter controls requirements for usage of lowercase letters in a password. When set to a negative number, any password will be required to contain that many lowercase characters. When set to a positive number, pam_cracklib will grant +1 additional length credit for each lowercase character. Add lcredit=-1 after pam_cracklib.so to require use of a lowercase character in passwords. Requiring a minimum number of lowercase characters makes password guessing attacks more difficult by ensuring a larger search space. Pneumonia Diligence
NIST SP800-53 IA-5
DISA CCI 193
lowCCE-26615-5Set Password Strength Minimum Different CharactersThe pam_cracklib module's difok parameter controls requirements for usage of different characters during a password change. Add difok=NUM after pam_cracklib.so to require differing characters when changing passwords, substituting NUM appropriately. The DoD requirement is 4. Requiring a minimum number of different characters during password changes ensures that newly changed passwords should not resemble previously compromised ones. Note that passwords which are changed on compromised systems will still be compromised, however. Bruising Kindness
NIST SP800-53 IA-5
DISA CCI 195
Set Lockouts for Failed Password AttemptsThe pam_faillock PAM module provides the capability to lock out user accounts after a number of failed login attempts. Its documentation is available in /usr/share/doc/pam-VERSION/txts/README.pam_faillock.
mediumCCE-26844-1Set Deny For Failed Password Attempts To configure the system to lock out accounts after a number of incorrect login attempts using pam_faillock.so: Add the following lines immediately below the pam_env.so statement in /etc/pam.d/system-auth: auth [default=die] pam_faillock.so authfail deny=3 unlock_time=604800 fail_interval=900 auth required pam_faillock.so authsucc deny=3 unlock_time=604800 fail_interval=900 Locking out user accounts after a number of incorrect attempts prevents direct password guessing attacks. Diabetes Chastity
NIST SP800-53 AC-7(a)
DISA CCI 44
mediumCCE-3410-8Set Lockout Time For Failed Password Attempts To configure the system to lock out accounts after a number of incorrect login attempts and require an administrator to unlock the account using pam_faillock.so: Add the following lines immediately below the pam_env.so statement in /etc/pam.d/system-auth: auth [default=die] pam_faillock.so authfail deny=3 unlock_time=604800 fail_interval=900 auth required pam_faillock.so authsucc deny=3 unlock_time=604800 fail_interval=900 Locking out user accounts after a number of incorrect attempts prevents direct password guessing attacks. Ensuring that an administrator is involved in unlocking locked accounts draws appropriate attention to such situations. Pain Temperance
NIST SP800-53 AC-7(b)
DISA CCI 47
mediumCCE-3410-8Set Interval For Counting Failed Password Attempts To configure the system to lock out accounts after a number of incorrect login attempts within a 15 minute interval using pam_faillock.so: Add the following lines immediately below the pam_env.so statement in /etc/pam.d/system-auth: auth [default=die] pam_faillock.so authfail deny=3 unlock_time=604800 fail_interval=900 auth required pam_faillock.so authsucc deny=3 unlock_time=604800 fail_interval=900 Locking out user accounts after a number of incorrect attempts within a specific period of time prevents direct password guessing attacks. Muscle Soreness Temperance
NIST SP800-53 AC-7(a)
DISA CCI 1452
mediumCCE-26741-9Limit Password ReuseDo not allow users to reuse recent passwords. This can be accomplished by using the remember option for the pam_unix PAM module. In the file /etc/pam.d/system-auth, append remember=24 to the line which refers to the pam_unix.so module, as shown: password sufficient pam_unix.so existing_options remember=24 The DoD requirement is 24 passwords. Preventing re-use of previous passwords helps ensure that a compromised password is not re-used by a user. Poison Ivy Charity
NIST SP800-53 IA-5
DISA CCI 200
Set Password Hashing AlgorithmThe system's default algorithm for storing password hashes in /etc/shadow is SHA-512. This can be configured in several locations.
mediumCCE-26303-8Set Password Hashing Algorithm in /etc/pam.d/system-auth In /etc/pam.d/system-auth, the password section of the file controls which PAM modules execute during a password change. Set the pam_unix.so module in the password section to include the argument sha512, as shown below: password sufficient pam_unix.so sha512 other arguments... This will help ensure when local users change their passwords, hashes for the new passwords will be generated using the SHA-512 algorithm. This is the default. Using a stronger hashing algorithm makes password cracking attacks more difficult. Dehydration Temperance
NIST SP800-53 IA-5
DISA CCI 803
mediumCCE-TODOSet Password Hashing Algorithm in /etc/login.defs In /etc/login.defs, add or correct the following line to ensure the system will use SHA-512 as the hashing algorithm: ENCRYPT_METHOD SHA512 Using a stronger hashing algorithm makes password cracking attacks more difficult. Bad Breath Patience
NIST SP800-53 IA-5
DISA CCI 803
mediumCCE-TODOSet Password Hashing Algorithm in /etc/libuser.conf In /etc/libuser.conf, add or correct the following line in its [defaults] section to ensure the system will use the SHA-512 algorithm for password hashing: crypt_style = sha512 Using a stronger hashing algorithm makes password cracking attacks more difficult. Chickenpox Patience
NIST SP800-53 IA-5
DISA CCI 803
Secure Session Configuration Files for Login AccountsWhen a user logs into a Unix account, the system configures the user's session by reading a number of files. Many of these files are located in the user's home directory, and may have weak permissions as a result of user error or misconfiguration. If an attacker can modify or even read certain types of account configuration information, they can often gain full access to the affected user's account. Therefore, it is important to test and correct configuration file permissions for interactive accounts, particularly those of privileged users such as root or system administrators.
lownsLimit the Number of Concurrent Login Sessions Allowed Per User Limiting the number of allowed users and sessions per user can limit risks related to Denial of Service attacks. This addresses concurrent sessions for a single account and does not address concurrent sessions by a single user via multiple accounts. To set the number of concurrent sessions per user add the following line in /etc/security/limits.conf: * hard maxlogins MAX Where MAX is the maximum number of login sessions allowed. Limiting simultaneous user logins can insulate the system from denial of service problems caused by excessive logins. Automated login processes operating improperly or maliciously may result in an exceptional number of simultaneous login sessions. Influenza Humility
DISA CCI 54
Ensure that No Dangerous Directories Exist in Root's PathThe active path of the root account can be obtained by starting a new root shell and running: # echo $PATH This will produce a colon-separated list of directories in the path. Certain path elements could be considered dangerous, as they could lead to root executing unknown or untrusted programs, which could contain malicious code. Since root may sometimes work inside untrusted directories, the . character, which represents the current directory, should never be in the root path, nor should any directory which can be written to by an unprivileged or semi-privileged (system) user. It is a good practice for administrators to always execute privileged commands by typing the full path to the command.
lowCCE-26826-8Ensure that Root's Path Does Not Include Relative Paths or Null Directories Ensure that none of the directories in root's path is equal to a single . character, or that it contains any instances that lead to relative path traversal, such as .. or beginning a path without the slash (/) character. Also ensure that there are no "empty" elements in the path, such as in these examples: PATH=:/bin PATH=/bin: PATH=/bin::/sbin These empty elements have the same effect as a single . character. Including these entries increases the risk that root could execute code from an untrusted location. Cramps Diligence
NIST SP800-53
lowCCE-26768-2Ensure that Root's Path Does Not Include World or Group-Writable Directories For each element in root's path, run: # ls -ld DIR and ensure that write permissions are disabled for group and other. Such entries increase the risk that root could execute code provided by unprivileged users, and potentially malicious code. Poison Sumac Chastity
NIST SP800-53
lowCCE-26981-1Ensure that User Home Directories are not Group-Writable or World-ReadableFor each human user USER of the system, view the permissions of the user's home directory: # ls -ld /home/USER Ensure that the directory is not group-writable and that it is not world-readable. If necessary, repair the permissions: # chmod g-w /home/USER # chmod o-rwx /home/USER User home directories contain many configuration files which affect the behavior of a user's account. No user should ever have write permission to another user's home directory. Group shared directories can be configured in sub-directories or elsewhere in the filesystem if they are needed. Typically, user home directories should not be world-readable, as it would disclose file names to other users. If a subset of users need read access to one another's home directories, this can be provided using groups or ACLs. Hiccups Patience
NIST SP800-53
Ensure that Users Have Sensible Umask Values The umask setting controls the default permissions for the creation of new files. With a default umask setting of 077, files and directories created by users will not be readable by any other user on the system. Users who wish to make specific files group- or world-readable can accomplish this by using the chmod command. Additionally, users can make all their files readable to their group by default by setting a umask of 027 in their shell configuration files. If default per-user groups exist (that is, if every user has a default group whose name is the same as that user's username and whose only member is the user), then it may even be safe for users to select a umask of 007, making it very easy to intentionally share files with groups of which the user is a member.
lowCCE-26917-5Ensure the Default Bash Umask is Set Correctly To ensure the default umask for users of the Bash shell is set properly, add or correct the umask setting in /etc/bashrc to read as follows: umask 077 The umask value influences the permissions assigned to files when they are created. A misconfigured umask value could result in files with excessive permissions that can be read and/or written to by unauthorized users. Sunburn Skin Diligence
NIST SP800-53
lowCCE-27034-8Ensure the Default C Shell Umask is Set Correctly To ensure the default umask for users of the C shell is set properly, add or correct the umask setting in /etc/csh.cshrc to read as follows: umask 077 The umask value influences the permissions assigned to files when they are created. A misconfigured umask value could result in files with excessive permissions that can be read and/or written to by unauthorized users. Dehydration Kindness
NIST SP800-53
lowCCE-26669-2Ensure the Default Umask is Set Correctly in /etc/profile To ensure the default umask controlled by /etc/profile is set properly, add or correct the umask setting in /etc/profile to read as follows: umask 077 The umask value influences the permissions assigned to files when they are created. A misconfigured umask value could result in files with excessive permissions that can be read and/or written to by unauthorized users. Halitosis Chastity
NIST SP800-53
lowCCE-26371-5Ensure the Default Umask is Set Correctly in login.defs To ensure the default umask controlled by /etc/login.defs is set properly, add or correct the umask setting in /etc/login.defs to read as follows: umask 077 The umask value influences the permissions assigned to files when they are created. A misconfigured umask value could result in files with excessive permissions that can be read and/or written to by unauthorized users. Diabetes Patience
NIST SP800-53
Protect Physical Console AccessIt is impossible to fully protect a system from an attacker with physical access, so securing the space in which the system is located should be considered a necessary step. However, there are some steps which, if taken, make it more difficult for an attacker to quickly or undetectably modify a system from its console.
Set Boot Loader PasswordDuring the boot process, the boot loader is responsible for starting the execution of the kernel and passing options to it. The boot loader allows for the selection of different kernels - possibly on different partitions or media. The default RHEL boot loader for x86 systems is called GRUB. Options it can pass to the kernel include single-user mode, which provides root access without any authentication, and the ability to disable SELinux. To prevent local users from modifying the boot parameters and endangering security, protect the boot loader configuration with a password and ensure its configuration file's permissions are set properly.
mediumCCE-26995-1Verify /etc/grub.conf User OwnershipThe file /etc/grub.conf should be owned by the root user to prevent destruction or modification of the file. To properly set the owner of /etc/grub.conf, run the command: # chown root /etc/grub.conf Only root should be able to modify important boot parameters. Stinging Nettle Temperance
NIST SP800-53
mediumCCE-27022-3Verify /etc/grub.conf Group OwnershipThe file /etc/grub.conf should be group-owned by the root group to prevent destruction or modification of the file. To properly set the group owner of /etc/grub.conf, run the command: # chgrp root /etc/grub.conf The root group is a highly-privileged group. Furthermore, the group-owner of this file should not have any access privileges anyway. Pink Eye Diligence
NIST SP800-53
mediumCCE-26949-8Verify /etc/grub.conf PermissionsFile permissions for /etc/grub.conf should be set to 600, which is the default. To properly set the permissions of /etc/grub.conf, run the command: # chmod 600 /etc/grub.conf Proper permissions ensure that only the root user can modify important boot parameters. High Cholesterol Charity
NIST SP800-53
mediumCCE-26911-8Set Boot Loader PasswordThe grub boot loader should have password protection enabled to protect boot-time settings. To do so, select a password and then generate a hash from it by running the following command: # grub-crypt --sha-512 When prompted to enter a password, insert the following line into /etc/grub.conf immediately after the header comments. (Use the output from grub-crypt as the value of password-hash): password --encrypted password-hash Password protection on the boot loader configuration ensures users with physical access cannot trivially alter important bootloader settings. These include which kernel to use, and whether to enter single-user mode. Poison Oak Temperance
NIST SP800-53 IA-5
DISA CCI 213
mediumCCE-27040-5Require Authentication for Single User ModeSingle-user mode is intended as a system recovery method, providing a single user root access to the system by providing a boot option at startup. By default, no authentication is performed if single-user mode is selected. To require entry of the root password even if the system is started in single-user mode, add or correct the following line in the file /etc/sysconfig/init: SINGLE=/sbin/sulogin This prevents attackers with physical access from trivially bypassing security on the machine and gaining root access. Such accesses are further prevented by configuring the bootloader password. Muscle Cramping Chastity
NIST SP800-53 IA-5
DISA CCI 213
highnsDisable Ctrl-Alt-Del Reboot Activation By default, the system includes the following line in /etc/init/control-alt-delete to reboot the system when the Ctrl-Alt-Del key sequence is pressed: exec /sbin/shutdown -r now "Control-Alt-Delete pressed" To configure the system to log a message instead of rebooting the system, alter that line to read as follows: exec /usr/bin/logger -p security.info "Control-Alt-Delete pressed" A locally logged-in user who presses Ctrl-Alt-Del, when at the console, can reboot the system. If accidentally pressed, as could happen in the case of mixed OS environment, this can create the risk of short-term loss of availability of systems due to unintentional reboot. In the GNOME graphical environment, risk of unintentional reboot from the Ctrl-Alt-Del sequence is reduced because the user will be prompted before any action is taken. Fungal Infections Patience
mediumCCE-27043-9Disable Interactive Boot To disable the ability for users to perform interactive startups, edit the file /etc/sysconfig/init. Add or correct the line: PROMPT=no The PROMPT option allows the console user to perform an interactive system startup, in which it is possible to select the set of services which are started on boot. Using interactive boot, the console user could disable auditing, firewalls, or other services, weakening system security. Pneumonia Patience
NIST SP800-53 IA-4, SC-2
DISA CCI 213
Configure Screen LockingWhen a user must temporarily leave an account logged-in, screen locking should be employed to prevent passersby from abusing the account. User education and training is particularly important for screen locking to be effective, and policies can be implemented to reinforce this. Automatic screen locking is only meant as a safeguard for those cases where a user forgot to lock the screen.
Configure GUI Screen LockingIn the default GNOME desktop, the screen can be locked by choosing Lock Screen from the System menu. The gconftool-2 program can be used to enforce mandatory screen locking settings for the default GNOME environment. The following sections detail commands to enforce idle activation of the screen saver, screen locking, a blank-screen screensaver, and an idle activation time. Because users should be trained to lock the screen when they step away from the computer, the automatic locking feature is only meant as a backup. The Lock Screen icon from the System menu can also be dragged to the taskbar in order to facilitate even more convenient screen-locking. The root account cannot be screen-locked, but this should have no practical effect as the root account should never be used to log into an X Windows environment, and should only be used to for direct login via console in emergency circumstances. For more information about configuring GNOME screensaver, see http://live.gnome.org/GnomeScreensaver. For more information about enforcing preferences in the GNOME environment using the GConf configuration system, see http://projects.gnome.org/gconf and the man page gconftool-2(1).
mediumCCE-26828-4Set GNOME Login Inactivity Timeout Run the following command to set the idle time-out value for inactivity in the GNOME desktop to 15 minutes: # gconftool-2 \ --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type int \ --set /apps/gnome-screensaver/idle_delay 15 Setting the idle delay controls when the screensaver will start, and can be combined with screen locking to prevent access from passersby. Diabetes Temperance
NIST SP800-53 AC-11(a)
DISA CCI 57
mediumCCE-26600-7GNOME Desktop Screensaver Mandatory Use Run the following command to activate the screensaver in the GNOME desktop after a period of inactivity: # gconftool-2 --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type bool \ --set /apps/gnome-screensaver/idle_activation_enabled true Enabling idle activation of the screen saver ensures the screensaver will be activated after the idle delay. Applications requiring continuous, real-time screen display (such as network management products) require the login session does not have administrator rights and the display station is located in a controlled-access area. Muscle Aches Patience
NIST SP800-53 AC-11(a)
DISA CCI 57
mediumCCE-26235-2Enable Screen Lock Activation After Idle Period Run the following command to activate locking of the screensaver in the GNOME desktop when it is activated: # gconftool-2 --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type bool \ --set /apps/gnome-screensaver/lock_enabled true Enabling the activation of the screen lock after an idle period ensures password entry will be required in order to access the system, preventing access by passersby. Psoriasis Charity
NIST SP800-53 AC-11(a)
DISA CCI 57
lowCCE-26638-7Implement Blank Screen Saver Run the following command to set the screensaver mode in the GNOME desktop to a blank screen: # gconftool-2 --direct \ --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \ --type string \ --set /apps/gnome-screensaver/mode blank-only Setting the screensaver mode to blank-only conceals the contents of the display from passersby. Red Eyes Temperance
NIST SP800-53 AC-11(b)
DISA CCI 60
Configure Console Screen Locking A console screen locking mechanism is provided in the screen package, which is not installed by default.
lowCCE-26940-7Install the screen Package To enable console screen locking, install the screen package: # yum install screen Instruct users to begin new terminal sessions with the following command: $ screen The console can now be locked with the following key combination: ctrl+a x Installing screen ensures a console locking capability is available for users who may need to suspend console logins. Gallstones Patience
NIST SP800-53
Hardware Tokens for Authentication The use of hardware tokens such as smart cards for system login provides stronger, two-factor authentication than using a username/password. In Red Hat Enterprise Linux servers and workstations, hardware token login is not enabled by default and must be enabled in the system settings.
mediumnsEnable Smart Card Login To enable smart card authentication, consult the documentation at: https://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Managing_Smart_Cards/enabling-smart-card-login.html Smart card login provides two-factor authentication stronger than that provided by a username/password combination. Smart cards leverage a PKI (public key infrastructure) in order to provide and verify credentials. Pain Temperance
DISA CCI 765, 766, 767, 768, 771, 772, 884
Warning Banners for System AccessesEach system should expose as little information about itself as possible. System banners, which are typically displayed just before a login prompt, give out information about the service or the host's operating system. This might include the distribution name and the system kernel version, and the particular version of a network service. This information can assist intruders in gaining access to the system as it can reveal whether the system is running vulnerable software. Most network services can be configured to limit what information is displayed. Many organizations implement security policies that require a system banner provide notice of the system's ownership, provide warning to unauthorized users, and remind authorized users of their consent to monitoring.
mediumCCE-26974-6Modify the System Login Banner To configure the system login banner: Edit /etc/issue. Replace the default text with a message compliant with the local site policy or a legal disclaimer. The DoD required text is either: You are accessing a U.S. Government (USG) Information System (IS) that is provided for USG-authorized use only. By using this IS (which includes any device attached to this IS), you consent to the following conditions: -The USG routinely intercepts and monitors communications on this IS for purposes including, but not limited to, penetration testing, COMSEC monitoring, network operations and defense, personnel misconduct (PM), law enforcement (LE), and counterintelligence (CI) investigations. -At any time, the USG may inspect and seize data stored on this IS. -Communications using, or data stored on, this IS are not private, are subject to routine monitoring, interception, and search, and may be disclosed or used for any USG-authorized purpose. -This IS includes security measures (e.g., authentication and access controls) to protect USG interests -- not for your personal benefit or privacy. -Notwithstanding the above, using this IS does not constitute consent to PM, LE or CI investigative searching or monitoring of the content of privileged communications, or work product, related to personal representation or services by attorneys, psychotherapists, or clergy, and their assistants. Such communications and work product are private and confidential. See User Agreement for details. OR: I've read & consent to terms in IS user agreem't. An appropriate warning message reinforces policy awareness during the logon process and facilitates possible legal action against attackers. Ingrown Toenails Chastity
NIST SP800-53 AC-8(a), AC-8(c), AC-8
DISA CCI 48, 1384, 1385, 1386, 1387, 1388
Implement a GUI Warning BannerIn the default graphical environment, users logging directly into the system are greeted with a login screen provided by the GNOME Display Manager (GDM). The warning banner should be displayed in this graphical environment for these users. The following sections describe how to configure the GDM login banner.
mediumCCE-27195-7Enable GUI Warning Banner To enable displaying a login warning banner in the GNOME Display Manager's login screen, run the following command: sudo -u gdm gconftool-2 \ --type bool \ --set /apps/gdm/simple-greeter/banner_message_enable true To display a banner, this setting must be enabled and then banner text must also be set. An appropriate warning message reinforces policy awareness during the logon process and facilitates possible legal action against attackers. Constipation Humility
NIST SP800-53 AC-8(a), AC-8(c)
DISA CCI 48, 50
mediumCCE-27017-3Set GUI Warning Banner Text To set the text shown by the GNOME Display Manager in the login screen, run the following command: sudo -u gdm gconftool-2 \ --type string \ --set /apps/gdm/simple-greeter/banner_message_text \ "Text of the warning banner here" When entering a warning banner that spans several lines, remember to begin and end the string with ". This command writes directly to the file /var/lib/gdm/.gconf/apps/gdm/simple-greeter/%gconf.xml, and this file can later be edited directly if necessary. An appropriate warning message reinforces policy awareness during the logon process and facilitates possible legal action against attackers. Pink Eye Chastity
NIST SP800-53 AC-8(a), AC-8(c)
DISA CCI 48, 1384, 1385, 1386, 1387, 1388
lowCCE-TODODisable the User ListIn the default graphical environment, users logging directly into the system are greeted with a login screen that displays all known users. This functionality should be disabled. Run the following command to disable the user list: sudo -u gdm gconftool-2 \ --type bool \ --set /apps/gdm/simple-greeter/disable_user_list true Leaving the user list enabled is a security risk since it allows anyone with physical access to the system to quickly enumerate known user accounts without logging in. Bee Stings Charity
NIST SP800-53 AC-23
Network Configuration and FirewallsMost machines must be connected to a network of some sort, and this brings with it the substantial risk of network attack. This section discusses the security impact of decisions about networking which must be made when configuring a system. This section also discusses firewalls, network access controls, and other network security frameworks, which allow system-level rules to be written that can limit an attackers' ability to connect to your system. These rules can specify that network traffic should be allowed or denied from certain IP addresses, hosts, and networks. The rules can also specify which of the system's network services are available to particular hosts or networks.
Disable Unused InterfacesNetwork interfaces expand the attack surface of the system. Unused interfaces are not monitored or controlled, and should be disabled. If the system does not require network communications but still needs to use the loopback interface, remove all files of the form ifcfg-interface except for ifcfg-lo from /etc/sysconfig/network-scripts: # rm /etc/sysconfig/network-scripts/ifcfg-interface If the system is a standalone machine with no need for network access or even communication over the loopback device, then disable this service. The network service can be disabled with the following command: # chkconfig network off
lowCCE-27151-0Disable Zeroconf NetworkingZeroconf networking allows the system to assign itself an IP address and engage in IP communication without a statically-assigned address or even a DHCP server. Automatic address assignment via Zeroconf (or DHCP) is not recommended. To disable Zeroconf automatic route assignment in the 169.245.0.0 subnet, add or correct the following line in /etc/sysconfig/network: NOZEROCONF=yes Zeroconf addresses are in the network 169.254.0.0. The networking scripts add entries to the system's routing table for these addresses. Zeroconf address assignment commonly occurs when the system is configured to use DHCP but fails to receive an address assignment from the DHCP server. Migraine Patience
NIST SP800-53 CM-7
lowCCE-27152-8Ensure System is Not Acting as a Network SnifferThe system should not be acting as a network sniffer, which can capture all traffic on the network to which it is connected. Run the following to determine if any interface is running in promiscuous mode: $ ip link | grep PROMISC If any results are returned, then a sniffing process (such as tcpdump or Wireshark) is likely to be using the interface and this should be investigated. Pneumonia Charity
NIST SP800-53 CM-7, MA-3
Kernel Parameters Which Affect NetworkingThe sysctl utility is used to set parameters which affect the operation of the Linux kernel. Kernel parameters which affect networking and have security implications are described here.
Network Parameters for Hosts OnlyIf the system is not going to be used as a router, then setting certain kernel parameters ensure that the host will not perform routing of network traffic.
mediumCCE-27001-7Disable Kernel Parameter for Sending ICMP Redirects by Default To set the runtime status of the net.ipv4.conf.default.send_redirects kernel parameter, run the following command: # sysctl -w net.ipv4.conf.default.send_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.default.send_redirects = 0 Sending ICMP redirects permits the system to instruct other systems to update their routing information. The ability to send ICMP redirects is only appropriate for routers. Seasonal Affective Disorder Charity
NIST SP800-53 AC-4, CM-7, SC-5, SC-7
DISA CCI 1551
mediumCCE-27004-1Disable Kernel Parameter for Sending ICMP Redirects for All Interfaces To set the runtime status of the net.ipv4.conf.all.send_redirects kernel parameter, run the following command: # sysctl -w net.ipv4.conf.all.send_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.all.send_redirects = 0 Sending ICMP redirects permits the system to instruct other systems to update their routing information. The ability to send ICMP redirects is only appropriate for routers. Dehydration Temperance
NIST SP800-53 CM-7
DISA CCI 1551
mediumCCE-26866-4Disable Kernel Parameter for IP Forwarding To set the runtime status of the net.ipv4.ip_forward kernel parameter, run the following command: # sysctl -w net.ipv4.ip_forward=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.ip_forward = 0 IP forwarding permits the kernel to forward packets from one network interface to another. The ability to forward packets between two networks is only appropriate for routers. Upset Stomach Chastity
NIST SP800-53 CM-7, SC-5
DISA CCI 366
Network Related Kernel Runtime Parameters for Hosts and RoutersCertain kernel parameters should be set for systems which are acting as either hosts or routers to improve the system's ability defend against certain types of IPv4 protocol attacks.
mediumCCE-27037-1Disable Kernel Parameter for Accepting Source-Routed Packets for All Interfaces To set the runtime status of the net.ipv4.conf.all.accept_source_route kernel parameter, run the following command: # sysctl -w net.ipv4.conf.all.accept_source_route=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.all.accept_source_route = 0 Accepting source-routed packets in the IPv4 protocol has few legitimate uses. It should be disabled unless it is absolutely required. Sprain Charity
NIST SP800-53 CM-7
DISA CCI 1551
mediumCCE-27027-2Disable Kernel Parameter for Accepting ICMP Redirects for All Interfaces To set the runtime status of the net.ipv4.conf.all.accept_redirects kernel parameter, run the following command: # sysctl -w net.ipv4.conf.all.accept_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.all.accept_redirects = 0 Accepting ICMP redirects has few legitimate uses. It should be disabled unless it is absolutely required. Canker Sores Diligence
NIST SP800-53 CM-7
DISA CCI 1503, 1551
mediumCCE-26854-0Disable Kernel Parameter for Accepting Secure Redirects for All Interfaces To set the runtime status of the net.ipv4.conf.all.secure_redirects kernel parameter, run the following command: # sysctl -w net.ipv4.conf.all.secure_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.all.secure_redirects = 0 Accepting "secure" ICMP redirects (from those gateways listed as default gateways) has few legitimate uses. It should be disabled unless it is absolutely required. Tooth Ache Humility
NIST SP800-53 AC-4, CM-7
DISA CCI 1503, 1551
lowCCE-27066-0Enable Kernel Parameter to Log Martian Packets To set the runtime status of the net.ipv4.conf.all.log_martians kernel parameter, run the following command: # sysctl -w net.ipv4.conf.all.log_martians=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.all.log_martians = 1 The presence of "martian" packets (which have impossible addresses) as well as spoofed packets, source-routed packets, and redirects could be a sign of nefarious network activity. Logging these packets enables this activity to be detected. Rheumatoid Arthritis Patience
NIST SP800-53 AC-17(7), CM-7
DISA CCI 126
mediumCCE-26983-7Disable Kernel Parameter for Accepting Source-Routed Packets By Default To set the runtime status of the net.ipv4.conf.default.accept_source_route kernel parameter, run the following command: # sysctl -w net.ipv4.conf.default.accept_source_route=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.default.accept_source_route = 0 Accepting source-routed packets in the IPv4 protocol has few legitimate uses. It should be disabled unless it is absolutely required. Black Eye Patience
NIST SP800-53 AC-4, CM-7, SC-5, SC-7
DISA CCI 1551
lowCCE-27015-7Disable Kernel Parameter for Accepting ICMP Redirects By Default To set the runtime status of the net.ipv4.conf.default.accept_redirects kernel parameter, run the following command: # sysctl -w net.ipv4.conf.default.accept_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.default.accept_redirects = 0 This feature of the IPv4 protocol has few legitimate uses. It should be disabled unless it is absolutely required. Eczema Kindness
NIST SP800-53 AC-4, CM-7, SC-5, SC-7
DISA CCI 1551
mediumCCE-26831-8Disable Kernel Parameter for Accepting Secure Redirects By Default To set the runtime status of the net.ipv4.conf.default.secure_redirects kernel parameter, run the following command: # sysctl -w net.ipv4.conf.default.secure_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.default.secure_redirects = 0 Accepting "secure" ICMP redirects (from those gateways listed as default gateways) has few legitimate uses. It should be disabled unless it is absolutely required. Sunburn Skin Humility
NIST SP800-53 AC-4, CM-7, SC-5, SC-7
DISA CCI 1551
lowCCE-26883-9Enable Kernel Parameter to Ignore ICMP Broadcast Echo Requests To set the runtime status of the net.ipv4.icmp_echo_ignore_broadcasts kernel parameter, run the following command: # sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.icmp_echo_ignore_broadcasts = 1 Ignoring ICMP echo requests (pings) sent to broadcast or multicast addresses makes the system slightly more difficult to enumerate on the network. Stress Temperance
NIST SP800-53 CM-7, SC-5
DISA CCI 1551
lowCCE-26993-6Enable Kernel Parameter to Ignore Bogus ICMP Error Responses To set the runtime status of the net.ipv4.icmp_ignore_bogus_error_responses kernel parameter, run the following command: # sysctl -w net.ipv4.icmp_ignore_bogus_error_responses=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.icmp_ignore_bogus_error_responses = 1 Ignoring bogus ICMP error responses reduces log size, although some activity would not be logged. Snake Bite Diligence
NIST SP800-53 CM-7, SC-5
mediumCCE-27053-8Enable Kernel Parameter to Use TCP Syncookies To set the runtime status of the net.ipv4.tcp_syncookies kernel parameter, run the following command: # sysctl -w net.ipv4.tcp_syncookies=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.tcp_syncookies = 1 A TCP SYN flood attack can cause a denial of service by filling a system's TCP connection table with connections in the SYN_RCVD state. Syncookies can be used to track a connection when a subsequent ACK is received, verifying the initiator is attempting a valid connection and is not a flood source. This feature is activated when a flood condition is detected, and enables the system to continue servicing valid connection requests. Cuts Humility
NIST SP800-53 AC-4
DISA CCI 1092, 1095
mediumCCE-26979-5Enable Kernel Parameter to Use Reverse Path Filtering for All Interfaces To set the runtime status of the net.ipv4.conf.all.rp_filter kernel parameter, run the following command: # sysctl -w net.ipv4.conf.all.rp_filter=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.all.rp_filter = 1 Enabling reverse path filtering drops packets with source addresses that should not have been able to be received on the interface they were received on. It should not be used on systems which are routers for complicated networks, but is helpful for end hosts and routers serving small networks. Gallstones Humility
NIST SP800-53 AC-4, SC-5, SC-7
DISA CCI 1551
mediumCCE-26915-9Enable Kernel Parameter to Use Reverse Path Filtering by Default To set the runtime status of the net.ipv4.conf.default.rp_filter kernel parameter, run the following command: # sysctl -w net.ipv4.conf.default.rp_filter=1 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv4.conf.default.rp_filter = 1 Enabling reverse path filtering drops packets with source addresses that should not have been able to be received on the interface they were received on. It should not be used on systems which are routers for complicated networks, but is helpful for end hosts and routers serving small networks. Hives Patience
NIST SP800-53 AC-4, SC-5, SC-7
Wireless NetworkingWireless networking, such as 802.11 (WiFi) and Bluetooth, can present a security risk to sensitive or classified systems and networks. Wireless networking hardware is much more likely to be included in laptop or portable systems than desktops or servers. Removal of hardware provides the greatest assurance that the wireless capability remains disabled. Acquisition policies often include provisions to prevent the purchase of equipment that will be used in sensitive spaces and includes wireless capabilities. If it is impractical to remove the wireless hardware, and policy permits the device to enter sensitive spaces as long as wireless is disabled, efforts should instead focus on disabling wireless capability via software.
Disable Wireless Through Software ConfigurationIf it is impossible to remove the wireless hardware from the device in question, disable as much of it as possible through software. The following methods can disable software support for wireless networking, but note that these methods do not prevent malicious software or careless users from re-activating the devices.
lowCCE-26878-9Disable WiFi or Bluetooth BIOSSome systems that include built-in wireless support offer the ability to disable the device through the BIOS. This is system-specific; consult your hardware manual or explore the BIOS setup during boot.Disabling wireless support in the BIOS prevents easy activation of the wireless interface, generally requiring administrators to reboot the system first. Bruising Temperance
NIST SP800-53 AC-18(a), AC-18(d), AC-18(3), CM-7
DISA CCI 85
lowCCE-27057-9Deactivate Wireless Network InterfacesDeactivating wireless network interfaces should prevent normal usage of the wireless capability. First, identify the interfaces available with the command: # ifconfig -a >Additionally,the following command may also be used to determine whether wireless support ('extensions') is included for a particular interface, though this may not always be a clear indicator: # iwconfig After identifying any wireless interfaces (which may have names like wlan0, ath0, wifi0, em1 or eth0), deactivate the interface with the command: # ifdown interface These changes will only last until the next reboot. To disable the interface for future boots, remove the appropriate interface file from /etc/sysconfig/network-scripts: # rm /etc/sysconfig/network-scripts/ifcfg-interface Wireless networking allows attackers within physical proximity to launch network-based attacks against systems, including those against local LAN protocols which were not designed with security in mind. Stress Diligence
NIST SP800-53 AC-18(a), AC-18(d), AC-18(3), CM-7
DISA CCI 85
mediumCCE-27081-9Disable Bluetooth Service The bluetooth service can be disabled with the following command: # chkconfig bluetooth off # service bluetooth stop Disabling the bluetooth service prevents the system from attempting connections to Bluetooth devices, which entails some security risk. Nevertheless, variation in this risk decision may be expected due to the utility of Bluetooth connectivity and its limited range. Headache Kindness
NIST SP800-53 AC-18(a), AC-18(d), AC-18(3), CM-7
DISA CCI 85, 1551
mediumCCE-26763-3Disable Bluetooth Kernel ModulesThe kernel's module loading system can be configured to prevent loading of the Bluetooth module. Add the following to the appropriate /etc/modprobe.d configuration file to prevent the loading of the Bluetooth module: install net-pf-31 /bin/false install bluetooth /bin/false If Bluetooth functionality must be disabled, preventing the kernel from loading the kernel module provides an additional safeguard against its activation. Hives Charity
NIST SP800-53 AC-18(a), AC-18(d), AC-18(3), CM-7
DISA CCI 85, 1551
IPv6The system includes support for Internet Protocol version 6. A major and often-mentioned improvement over IPv4 is its enormous increase in the number of available addresses. Another important feature is its support for automatic configuration of many network settings.
Disable Support for IPv6 unless Needed Despite configuration that suggests support for IPv6 has been disabled, link-local IPv6 address auto-configuration occurs even when only an IPv4 address is assigned. The only way to effectively prevent execution of the IPv6 networking stack is to instruct the system not to activate the IPv6 kernel module.
mediumCCE-27153-6Disable IPv6 Networking Support Automatic LoadingTo prevent the IPv6 kernel module (ipv6) from loading the IPv6 networking stack, add the following line to /etc/modprobe.d/disabled.conf (or another file in /etc/modprobe.d): options ipv6 disable=1 This permits the IPv6 module to be loaded (and thus satisfy other modules that depend on it), while disabling support for the IPv6 protocol. Any unnecessary network stacks - including IPv6 - should be disabled, to reduce the vulnerability to exploitation. Burns Patience
NIST SP800-53 CM-7
DISA CCI 1551
lowCCE-TODODisable Interface Usage of IPv6To prevent configuration of IPv6 for all interfaces, add or correct the following lines in /etc/sysconfig/network: NETWORKING_IPV6=no IPV6INIT=no For each network interface, add or correct the following lines in /etc/sysconfig/network-scripts/ifcfg-interface as an additional prevention mechanism: IPV6INIT=no Psoriasis Chastity
NIST SP800-53 CM-7
lowCCE-TODODisable Support for RPC IPv6RPC services for NFSv4 try to load transport modules for udp6 and tcp6 by default, even if IPv6 has been disabled in /etc/modprobe.d. To prevent RPC services such as rpc.mountd from attempting to start IPv6 network listeners, remove or comment out the following two lines in /etc/netconfig: udp6 tpi_clts v inet6 udp - - tcp6 tpi_cots_ord v inet6 tcp - - Muscle Aches Diligence
NIST SP800-53 CM-7
Configure IPv6 Settings if NecessaryA major feature of IPv6 is the extent to which systems implementing it can automatically configure their networking devices using information from the network. From a security perspective, manually configuring important configuration information is preferable to accepting it from the network in an unauthenticated fashion.
Disable Automatic ConfigurationDisable the system's acceptance of router advertisements and redirects by adding or correcting the following line in /etc/sysconfig/network (note that this does not disable sending router solicitations): IPV6_AUTOCONF=no
lowCCE-27164-3Disable Accepting IPv6 Router Advertisements To set the runtime status of the net.ipv6.conf.default.accept_ra kernel parameter, run the following command: # sysctl -w net.ipv6.conf.default.accept_ra=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv6.conf.default.accept_ra = 0 An illicit router advertisement message could result in a man-in-the-middle attack. Stinging Nettle Kindness
NIST SP800-53 CM-7
mediumCCE-27166-8Disable Accepting IPv6 Redirects To set the runtime status of the net.ipv6.conf.default.accept_redirects kernel parameter, run the following command: # sysctl -w net.ipv6.conf.default.accept_redirects=0 If this is not the system's default value, add the following line to /etc/sysctl.conf: net.ipv6.conf.default.accept_redirects = 0 An illicit ICMP redirect message could result in a man-in-the-middle attack. Burns Temperance
NIST SP800-53 CM-7
DISA CCI 1551
lowCCE-TODOManually Assign Global IPv6 AddressTo manually assign an IP address for an interface, edit the file /etc/sysconfig/network-scripts/ifcfg-interface. Add or correct the following line (substituting the correct IPv6 address): IPV6ADDR=2001:0DB8::ABCD/64 Manually assigning an IP address is preferable to accepting one from routers or from the network otherwise. The example address here is an IPv6 address reserved for documentation purposes, as defined by RFC3849. Hives Chastity
NIST SP800-53
lowCCE-27154-4Use Privacy Extensions for AddressTo introduce randomness into the automatic generation of IPv6 addresses, add or correct the following line in /etc/sysconfig/network-scripts/ifcfg-interface: IPV6_PRIVACY=rfc3041 Automatically-generated IPv6 addresses are based on the underlying hardware (e.g. Ethernet) address, and so it becomes possible to track a piece of hardware over its lifetime using its traffic. If it is important for a system's IP address to not trivially reveal its hardware address, this setting should be applied. Blisters Charity
NIST SP800-53
lowCCE-TODOManually Assign IPv6 Router AddressEdit the file /etc/sysconfig/network-scripts/ifcfg-interface, and add or correct the following line (substituting your gateway IP as appropriate): IPV6_DEFAULTGW=2001:0DB8::0001 Router addresses should be manually set and not accepted via any auto-configuration or router advertisement. Dehydration Humility
NIST SP800-53
lowCCE-27163-5Limit Network-Transmitted ConfigurationAdd the following lines to /etc/sysctl.conf to limit the configuration information requested from other systems, and accepted from the network: net.ipv6.conf.default.router_solicitations = 0 net.ipv6.conf.default.accept_ra_rtr_pref = 0 net.ipv6.conf.default.accept_ra_pinfo = 0 net.ipv6.conf.default.accept_ra_defrtr = 0 net.ipv6.conf.default.autoconf = 0 net.ipv6.conf.default.dad_transmits = 0 net.ipv6.conf.default.max_addresses = 1 The router_solicitations setting determines how many router solicitations are sent when bringing up the interface. If addresses are statically assigned, there is no need to send any solicitations. The accept_ra_pinfo setting controls whether the system will accept prefix info from the router. The accept_ra_defrtr setting controls whether the system will accept Hop Limit settings from a router advertisement. Setting it to 0 prevents a router from changing your default IPv6 Hop Limit for outgoing packets. The autoconf setting controls whether router advertisements can cause the system to assign a global unicast address to an interface. The dad_transmits setting determines how many neighbor solicitations to send out per address (global and link-local) when bringing up an interface to ensure the desired address is unique on the network. The max_addresses setting determines how many global unicast IPv6 addresses can be assigned to each interface. The default is 16, but it should be set to exactly the number of statically configured global addresses required. Parkinson's Disease Patience
NIST SP800-53 CM-7
IPTables and Ip6tablesA host-based firewall called Netfilter is included as part of the Linux kernel distributed with the system. It is activated by default. This firewall is controlled by the program iptables, and the entire capability is frequently referred to by this name. An analogous program called ip6tables handles filtering for IPv6. Unlike TCP Wrappers, which depends on the network server program to support and respect the rules written, Netfilter filtering occurs at the kernel level, before a program can even process the data from the network packet. As such, any program on the system is affected by the rules written. This section provides basic information about strengthening the iptables and ip6tables configurations included with the system. For more complete information that may allow the construction of a sophisticated ruleset tailored to your environment, please consult the references at the end of this section.
Inspect and Activate Default RulesView the currently-enforced iptables rules by running the command: # iptables -nL --line-numbers The command is analogous for the ip6tables program. If the firewall does not appear to be active (i.e., no rules appear), activate it and ensure that it starts at boot by issuing the following commands (and analogously for ip6tables): # service iptables restart The default iptables rules are: Chain INPUT (policy ACCEPT) num target prot opt source destination 1 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED 2 ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 3 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 4 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22 5 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited Chain FORWARD (policy ACCEPT) num target prot opt source destination 1 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited Chain OUTPUT (policy ACCEPT) num target prot opt source destination The ip6tables default rules are essentially the same.
mediumCCE-27006-6Verify ip6tables Enabled The ip6tables service can be enabled with the following command: # chkconfig ip6tables on The ip6tables service provides the system's host-based firewalling capability for IPv6 and ICMPv6. Stinging Nettle Kindness
NIST SP800-53 AC-4, CM-7
DISA CCI 32, 66, 1115, 1118, 1092, 1117, 1098, 1100, 1097, 1414
mediumCCE-27018-1Verify iptables Enabled The iptables service can be enabled with the following command: # chkconfig iptables on The iptables service provides the system's host-based firewalling capability for IPv4 and ICMP. Hives Charity
NIST SP800-53 AC-4, CM-7
DISA CCI 32, 66, 1115, 1118, 1092, 1117, 1098, 1100, 1097, 1414
Strengthen the Default RulesetThe default rules can be strengthened. The system scripts that activate the firewall rules expect them to be defined in the configuration files iptables and ip6tables in the directory /etc/sysconfig. Many of the lines in these files are similar to the command line arguments that would be provided to the programs /sbin/iptables or /sbin/ip6tables - but some are quite different. The following recommendations describe how to strengthen the default ruleset configuration file. An alternative to editing this configuration file is to create a shell script that makes calls to the iptables program to load in rules, and then invokes service iptables save to write those loaded rules to /etc/sysconfig/iptables. The following alterations can be made directly to /etc/sysconfig/iptables and /etc/sysconfig/ip6tables. Instructions apply to both unless otherwise noted. Language and address conventions for regular iptables are used throughout this section; configuration for ip6tables will be either analogous or explicitly covered.
mediumCCE-26444-0Set Default IPTables Policy for Incoming PacketsTo set the default policy to DROP (instead of ACCEPT) for the built-in INPUT chain which processes incoming packets, add or correct the following line in /etc/sysconfig/iptables: :INPUT DROP [0:0] In iptables the default policy is applied only after all the applicable rules in the table are examined for a match. Setting the default policy to DROP implements proper design for a firewall, i.e. any packets which are not explicitly permitted should not be accepted. Fever Patience
NIST SP800-53 CM-7
DISA CCI 66, 1109, 1154, 1414
mediumCCE-27186-6Set Default IPTables Policy for Forwarded PacketsTo set the default policy to DROP (instead of ACCEPT) for the built-in FORWARD chain which processes packets that will be forwarded from one interface to another, add or correct the following line in /etc/sysconfig/iptables: :FORWARD DROP [0:0] In iptables the default policy is applied only after all the applicable rules in the table are examined for a match. Setting the default policy to DROP implements proper design for a firewall, i.e. any packets which are not explicitly permitted should not be accepted. Muscle Aches Temperance
NIST SP800-53 CM-7
DISA CCI 1109
lownsRestrict ICMP Message TypesIn /etc/sysconfig/iptables, the accepted ICMP messages types can be restricted. To accept only ICMP echo reply, destination unreachable, and time exceeded messages, remove the line: -A INPUT -p icmp --icmp-type any -j ACCEPT and insert the lines: -A INPUT -p icmp --icmp-type echo-reply -j ACCEPT -A INPUT -p icmp --icmp-type destination-unreachable -j ACCEPT -A INPUT -p icmp --icmp-type time-exceeded -j ACCEPT To allow the system to respond to pings, also insert the following line: -A INPUT -p icmp --icmp-type echo-request -j ACCEPT Ping responses can also be limited to certain networks or hosts by using the -s option in the previous rule. Because IPv6 depends so heavily on ICMPv6, it is preferable to deny the ICMPv6 packets you know you don't need (e.g. ping requests) in /etc/sysconfig/ip6tables, while letting everything else through: -A INPUT -p icmpv6 --icmpv6-type echo-request -j DROP If you are going to statically configure the machine's address, it should ignore Router Advertisements which could add another IPv6 address to the interface or alter important network settings: -A INPUT -p icmpv6 --icmpv6-type router-advertisement -j DROP Restricting other ICMPv6 message types in /etc/sysconfig/ip6tables is not recommended because the operation- of IPv6 depends heavily on ICMPv6. Thus, more care must be taken when blocking ICMPv6 types. Bloody Nose Chastity
NIST SP800-53 CM-7
lownsLog and Drop Packets with Suspicious Source AddressesPackets with non-routable source addresses should be rejected, as they may indicate spoofing. Because the modified policy will reject non-matching packets, you only need to add these rules if you are interested in also logging these spoofing or suspicious attempts before they are dropped. If you do choose to log various suspicious traffic, add identical rules with a target of DROP after each LOG. To log and then drop these IPv4 packets, insert the following rules in /etc/sysconfig/iptables (excepting any that are intentionally used): -A INPUT -s 10.0.0.0/8 -j LOG --log-prefix "IP DROP SPOOF A: " -A INPUT -s 172.16.0.0/12 -j LOG --log-prefix "IP DROP SPOOF B: " -A INPUT -s 192.168.0.0/16 -j LOG --log-prefix "IP DROP SPOOF C: " -A INPUT -s 224.0.0.0/4 -j LOG --log-prefix "IP DROP MULTICAST D: " -A INPUT -s 240.0.0.0/5 -j LOG --log-prefix "IP DROP SPOOF E: " -A INPUT -d 127.0.0.0/8 -j LOG --log-prefix "IP DROP LOOPBACK: " Similarly, you might wish to log packets containing some IPv6 reserved addresses if they are not expected on your network: -A INPUT -i eth0 -s ::1 -j LOG --log-prefix "IPv6 DROP LOOPBACK: " -A INPUT -s 2002:E000::/20 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " -A INPUT -s 2002:7F00::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " -A INPUT -s 2002:0000::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " -A INPUT -s 2002:FF00::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " -A INPUT -s 2002:0A00::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " -A INPUT -s 2002:AC10::/28 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " -A INPUT -s 2002:C0A8::/32 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: " If you are not expecting to see site-local multicast or auto-tunneled traffic, you can log those: -A INPUT -s FF05::/16 -j LOG --log-prefix "IPv6 SITE-LOCAL MULTICAST: " -A INPUT -s ::0.0.0.0/96 -j LOG --log-prefix "IPv4 COMPATIBLE IPv6 ADDR: " If you wish to block multicasts to all link-local nodes (e.g. if you are not using router auto-configuration and do not plan to have any services that multicast to the entire local network), you can block the link-local all-nodes multicast address (before accepting incoming ICMPv6): -A INPUT -d FF02::1 -j LOG --log-prefix "Link-local All-Nodes Multicast: " However, if you're going to allow IPv4 compatible IPv6 addresses (of the form ::0.0.0.0/96), you should then consider logging the non-routable IPv4-compatible addresses: -A INPUT -s ::0.0.0.0/104 -j LOG --log-prefix "IP NON-ROUTABLE ADDR: " -A INPUT -s ::127.0.0.0/104 -j LOG --log-prefix "IP DROP LOOPBACK: " -A INPUT -s ::224.0.0.0.0/100 -j LOG --log-prefix "IP DROP MULTICAST D: " -A INPUT -s ::255.0.0.0/104 -j LOG --log-prefix "IP BROADCAST: " If you are not expecting to see any IPv4 (or IPv4-compatible) traffic on your network, consider logging it before it gets dropped: -A INPUT -s ::FFFF:0.0.0.0/96 -j LOG --log-prefix "IPv4 MAPPED IPv6 ADDR: " -A INPUT -s 2002::/16 -j LOG --log-prefix "IPv6 6to4 ADDR: " The following rule will log all traffic originating from a site-local address, which is deprecated address space: -A INPUT -s FEC0::/10 -j LOG --log-prefix "SITE-LOCAL ADDRESS TRAFFIC: " Bee Stings Patience
NIST SP800-53 AC-17, CM-7
Secure Sockets Layer Support The Secure Sockets Layer (SSL) protocol provides encrypted and authenticated network communications, and many network services include support for it. Using SSL is recommended, especially to avoid any plaintext transmission of sensitive data, even over a local network. The SSL implementation included with the system is called OpenSSL. Recent implementations of SSL may also be referred to as Transport Layer Security (TLS). SSL uses public key cryptography to provide authentication and encryption. Public key cryptography involves two keys, one called the public key and the other called the private key. These keys are mathematically related such that data encrypted with one key can only be decrypted by the other, and vice versa. As their names suggest, public keys can be distributed to anyone while a private key must remain known only to its owner. SSL uses certificates, which are files that hold cryptographic data: a public key, and a signature of that public key. In SSL authentication, a server presents a client with its certificate as a means of demonstrating that it is who it claims it is. If everything goes correctly, the client can verify the server's certificate by determining that the signature inside the certificate could only have been generated by a third party whom the client trusts. This third party is called a Certificate Authority (CA). Each client system should also have certificates from trusted CAs, and the client uses these CA certificates to verify the authenticity of the server's certificate. After authenticating a server using its certificate and a CA certificate, SSL provides encryption by using the server certificate to securely negotiate a shared secret key. If your server must communicate using SSL with systems that might not be able to securely accept a new CA certificate prior to any SSL communication, then paying an established CA (whose certificates your clients already have) to sign your server certificates is recommended. The steps for doing this vary by vendor. Once the signed certificates have been obtained, configuration of the services is the same whether they were purchased from a vendor or signed by your own CA. For setting up an internal network and encrypting local traffic, creating your own CA to sign SSL certificates can be appropriate. The major steps in this process are: Create a CA to sign certificatesCreate SSL certificates for servers using that CAEnable client support by distributing the CA's certificate
Create a CA to Sign CertificatesThe following instructions apply to OpenSSL since it is included with the system, but creating a CA is possible with any standards-compliant SSL toolkit. The security of certificates depends on the security of the CA that signed them, so performing these steps on a secure machine is critical. The system used as a CA should be physically secure and not connected to any network. It should receive any certificate signing requests (CSRs) via removable media and output certificates onto removable media. The script /etc/pki/tls/misc/CA is included to assist in the process of setting up a CA. This script uses many settings in /etc/pki/tls/openssl.cnf. The settings in this file can be changed to suit your needs and allow easier selection of default settings, particularly in the [req distinguished name] section. To create the CA: # cd /etc/pki/tls/misc # ./CA -newca When prompted, press enter to create a new CA key with the default name cakey.pem. When prompted, enter a password that will protect the private key, then enter the same password again to verify it. At the prompts, fill out as much of the CA information as is relevant for your site. You must specify a common name, or generation of the CA certificate will fail. Next, you will be prompted for the password, so that the script can re-open the private key in order to write the certificate. This step performs the following actions: creates the directory /etc/pki/CA (by default), which contains files necessary for the operation of a certificate authority. These are: serial, which contains the current serial number for certificates signed by the CAindex.txt, which is a text database file that contains information about certificates signedcrl, which is a directory for holding revoked certificatesprivate, a directory which stores the CA's private keycreates a public-private key pair for the CA in the file /etc/pki/CA/private/cakey.pem. The private key must be kept private in order to ensure the security of the certificates the CA will later sign.signs the public key (using the corresponding private key, in a process called self-signing) to create the CA certificate, which is then stored in /etc/pki/CA/cacert.pem. When the CA later signs a server certificate using its private key, it means that it is vouching for the authenticity of that server. A client can then use the CA's certificate (which contains its public key) to verify the authenticity of the server certificate.
NIST SP800-53 SC-12, SC-13
Create SSL Certificates for ServersCreating an SSL certificate for a server involves the following steps: A public-private key pair for the server must be generated.A certificate signing request (CSR) must be created from the key pair.The CSR must be signed by a certificate authority (CA) to create the server certificate. If a CA has been set up as described earlier, it can sign the CSR.The server certificate and keys must be installed on the server. Instructions on how to generate and sign SSL certificates are provided for the following common services can be found in the applicable sections of this guide: PostfixDovecotApache
NIST SP800-53 SC-12, SC-13
Enable Client SupportThe system ships with certificates from well-known commercial CAs. If your server certificates were signed by one of these established CAs, then this step is not necessary since the clients should include the CA certificate already. If your servers use certificates signed by your own CA, some user applications will warn that the server's certificate cannot be verified because the CA is not recognized. Other applications may simply fail to accept the certificate and refuse to operate, or continue operating without ever having properly verified the server certificate. To avoid this warning, and properly authenticate the servers, your CA certificate must be exported to every application on every client system that will be connecting to an SSL-enabled server.
Adding a Trusted CA for FirefoxTo import a new CA certificate into Firefox: Launch Firefox and choose Preferences from the menu.Click the Advanced button.Select the Encryption tab.Click the View Certificates button.Select the Authorities tab.Click the Import button at the bottom of the screen.Navigate to the CA certificate and import it. Determine whether the CA should be used to identify web sites, e-mail users, and software developers and trust it for each accordingly.
NIST SP800-53 AC-17, SC-12, SC-13
Adding a Trusted CA for ThunderbirdTo import a new CA certificate into Thunderbird: Launch Thunderbird and choose Preferences from the menu.Click the Advanced button.Select the Encryption tab.Click the View Certificates button.Select the Authorities tab.Click the Import button at the bottom of the screen.Navigate to the CA certificate and import it. Determine whether the CA should be used to identify web sites, e-mail users, and software developers and trust it for each accordingly.
NIST SP800-53 AC-17, SC-12, SC-13
Adding a Trusted CA for EvolutionTo import a new CA certificate into Evolution: Launch Evolution and choose Preferences from the menu.Click the Advanced button.Select the Encryption tab.Click the View Certificates button.Select the Authorities tab.Click the Import button at the bottom of the screen.Navigate to the CA certificate and import it. Determine whether the CA should be used to identify web sites, e-mail users, and software developers and trust it for each accordingly.
NIST SP800-53 AC-17, SC-12, SC-13
Remove Certificate Authorities, if AppropriateSurvey the certificate authorities trusted by Firefox, Thunderbird, Evolution, or other network clients. The list of certificate authorities for each program can be found via GUI, as described in the previous sections. Remove the certificate authorities which are not appropriate for your network connectivity needs. This may only make sense for some environments, and may create operational problems for a general purpose Internet-connected system.
NIST SP800-53 AC-17, SC-12, SC-13
Uncommon Network ProtocolsThe system includes support for several network protocols which are not commonly used. Although security vulnerabilities in kernel networking code are not frequently discovered, the consequences can be dramatic. Ensuring uncommon network protocols are disabled reduces the system's risk to attacks targeted at its implementation of those protocols.
mediumCCE-26448-1Disable DCCP Support The Datagram Congestion Control Protocol (DCCP) is a relatively new transport layer protocol, designed to support streaming media and telephony. To configure the system to prevent the dccp kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install dccp /bin/true Disabling DCCP protects the system against exploitation of any flaws in its implementation. Diaper Rash Charity
NIST SP800-53 CM-7
DISA CCI 382
mediumCCE-26410-1Disable SCTP Support The Stream Control Transmission Protocol (SCTP) is a transport layer protocol, designed to support the idea of message-oriented communication, with several streams of messages within one connection. To configure the system to prevent the sctp kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install sctp /bin/true Disabling SCTP protects the system against exploitation of any flaws in its implementation. Dehydration Chastity
NIST SP800-53 CM-7
DISA CCI 382
lowCCE-26239-4Disable RDS Support The Reliable Datagram Sockets (RDS) protocol is a transport layer protocol designed to provide reliable high- bandwidth, low-latency communications between nodes in a cluster. To configure the system to prevent the rds kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install rds /bin/true Disabling RDS protects the system against exploitation of any flaws in its implementation. Glaucoma Humility
NIST SP800-53 CM-7
DISA CCI 382
mediumCCE-26696-5Disable TIPC Support The Transparent Inter-Process Communication (TIPC) protocol is designed to provide communications between nodes in a cluster. To configure the system to prevent the tipc kernel module from being loaded, add the following line to a file in the directory /etc/modprobe.d: install tipc /bin/true Disabling TIPC protects the system against exploitation of any flaws in its implementation. Migraine Temperance
NIST SP800-53 CM-7
DISA CCI 382
IPSec SupportSupport for Internet Protocol Security (IPsec) is provided in RHEL 6 with Openswan.
lownsInstall openswan PackageThe Openswan package provides an implementation of IPsec and IKE, which permits the creation of secure tunnels over untrusted networks. The openswan package can be installed with the following command: # yum install openswan Providing the ability for remote users or systems to initiate a secure VPN connection protects information when it is transmitted over a wide area network. Pain Kindness
NIST SP800-53 AC-17, MA-4, SC-9
DISA CCI 1130, 1131
Configure SyslogThe syslog service has been the default Unix logging mechanism for many years. It has a number of downsides, including inconsistent log format, lack of authentication for received messages, and lack of authentication, encryption, or reliable transport for messages sent over a network. However, due to its long history, syslog is a de facto standard which is supported by almost all Unix applications. In RHEL 6, rsyslog has replaced ksyslogd as the syslog daemon of choice, and it includes some additional security features such as reliable, connection-oriented (i.e. TCP) transmission of logs, the option to log to database formats, and the encryption of log data en route to a central logging server. This section discusses how to configure rsyslog for best effect, and how to use tools provided with the system to maintain and monitor logs.
mediumCCE-26809-4Ensure rsyslog is Installed Rsyslog is installed by default. The rsyslog package can be installed with the following command: # yum install rsyslog The rsyslog package provides the rsyslog daemon, which provides system logging services. High Cholesterol Temperance
NIST SP800-53 AU-9
DISA CCI 1311, 1312
mediumCCE-26807-8Enable rsyslog ServiceThe rsyslog service provides syslog-style logging by default on RHEL 6. The rsyslog service can be enabled with the following command: # chkconfig rsyslog on The rsyslog service must be running in order to provide logging services, which are essential to system administration. Sty Patience
NIST SP800-53 AU-12
DISA CCI 1557, 1312, 1311
Ensure Proper Configuration of Log Files The file /etc/rsyslog.conf controls where log message are written. These are controlled by lines called rules, which consist of a selector and an action. These rules are often customized depending on the role of the system, the requirements of the environment, and whatever may enable the administrator to most effectively make use of log data. The default rules in RHEL 6 are: *.info;mail.none;authpriv.none;cron.none /var/log/messages authpriv.* /var/log/secure mail.* -/var/log/maillog cron.* /var/log/cron *.emerg * uucp,news.crit /var/log/spooler local7.* /var/log/boot.log See the man page rsyslog.conf(5) for more information. Note that the rsyslog daemon can be configured to use a timestamp format that some log processing programs may not understand. If this occurs, edit the file /etc/rsyslog.conf and add or edit the following line: $ ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat
lowCCE-26818-5Ensure Log Files Exist The log files written by rsyslog are determined by the second part of each rule line in /etc/rsyslog.conf. These typically all appear in /var/log. For any log file LOGFILE referenced in /etc/rsyslog.conf which does not already exist the following commands will create it and apply proper permissions: # touch LOGFILE # chown root:root LOGFILE # chmod 0600 LOGFILE If a log file referenced by rsyslog does not exist, rsyslog will not create it and important log messages can be lost. Influenza Temperance
NIST SP800-53
mediumCCE-26812-8Ensure Log Files Are Owned By Appropriate UserThe owner of all log files written by rsyslog should be root. These log files are determined by the second part of each Rule line in /etc/rsyslog.conf typically all appear in /var/log. For each log file LOGFILE referenced in /etc/rsyslog.conf, run the following command to inspect the file's owner: $ ls -l LOGFILE If the owner is not root, run the following command to correct this: # chown root LOGFILE The log files generated by rsyslog contain valuable information regarding system configuration, user authentication, and other such information. Log files should be protected from unauthorized access. Jaundice Charity
NIST SP800-53 AC-6
DISA CCI 1314
mediumCCE-26821-9Ensure Log Files Are Owned By Appropriate GroupThe group-owner of all log files written by rsyslog should be root. These log files are determined by the second part of each Rule line in /etc/rsyslog.conf and typically all appear in /var/log. For each log file LOGFILE referenced in /etc/rsyslog.conf, run the following command to inspect the file's group owner: $ ls -l LOGFILE If the owner is not root, run the following command to correct this: # chgrp root LOGFILE The log files generated by rsyslog contain valuable information regarding system configuration, user authentication, and other such information. Log files should be protected from unauthorized access. Sprain Charity
NIST SP800-53 AC-6
DISA CCI 1314
mediumCCE-27190-8Ensure System Log Files Have Correct PermissionsThe file permissions for all log files written by rsyslog should be set to 600, or more restrictive. These log files are determined by the second part of each Rule line in /etc/rsyslog.conf and typically all appear in /var/log. For each log file LOGFILE referenced in /etc/rsyslog.conf, run the following command to inspect the file's permissions: $ ls -l LOGFILE If the permissions are not 600 or more restrictive, run the following command to correct this: # chmod 0600 LOGFILE Log files can contain valuable information regarding system configuration. If the system log files are not protected unauthorized users could change the logged data, eliminating their forensic value. Cold Sore Charity
NIST SP800-53
Rsyslog Logs Sent To Remote Host If system logs are to be useful in detecting malicious activities, it is necessary to send logs to a remote server. An intruder who has compromised the root account on a machine may delete the log entries which indicate that the system was attacked before they are seen by an administrator. However, it is recommended that logs be stored on the local host in addition to being sent to the loghost, especially if rsyslog has been configured to use the UDP protocol to send messages over a network. UDP does not guarantee reliable delivery, and moderately busy sites will lose log messages occasionally, especially in periods of high traffic which may be the result of an attack. In addition, remote rsyslog messages are not authenticated in any way by default, so it is easy for an attacker to introduce spurious messages to the central log server. Also, some problems cause loss of network connectivity, which will prevent the sending of messages to the central server. For all of these reasons, it is better to store log messages both centrally and on each host, so that they can be correlated if necessary.
lowCCE-26801-1Ensure Logs Sent To Remote Host To configure rsyslog to send logs to a remote log server, open /etc/rsyslog.conf and read and understand the last section of the file, which describes the multiple directives necessary to activate remote logging. Along with these other directives, the system can be configured to forward its logs to a particular log server by adding or correcting one of the following lines, substituting loghost.example.com appropriately. The choice of protocol depends on the environment of the system; although TCP and RELP provide more reliable message delivery, they may not be supported in all environments. To use UDP for log message delivery: *.* @loghost.example.com To use TCP for log message delivery: *.* @@loghost.example.com To use RELP for log message delivery: *.* :omrelp:loghost.example.com A log server (loghost) receives syslog messages from one or more systems. This data can be used as an additional log source in the event a system is compromised and its local logs are suspect. Forwarding log messages to a remote loghost also provides system administrators with a centralized place to view the status of multiple hosts within the enterprise. Psoriasis Patience
NIST SP800-53 AU-3(2), AU-9
DISA CCI 1348, 136
Configure rsyslogd to Accept Remote Messages If Acting as a Log Server By default, RHEL6's rsyslog does not listen over the network for log messages. If needed, modules can be enabled to allow the rsyslog daemon to receive messages from other systems and for the system thus to act as a log server. If the machine is not a log server, then lines concerning these modules should remain commented out.
lowCCE-26803-7Ensure rsyslog Does Not Accept Remote Messages Unless Acting As Log ServerThe rsyslog daemon should not accept remote messages unless the system acts as a log server. To ensure that it is not listening on the network, ensure the following lines are not found in /etc/rsyslog.conf: $ModLoad imtcp.so $InputTCPServerRun port $ModLoad imudp.so $InputUDPServerRun port $ModLoad imrelp.so $InputRELPServerRun port Any process which receives messages from the network incurs some risk of receiving malicious messages. This risk can be eliminated for rsyslog by configuring it not to listen on the network. Upset Stomach Charity
NIST SP800-53 AU-9
lowCCE-TODOEnable rsyslog to Accept Messages via TCP, if Acting As Log ServerThe rsyslog daemon should not accept remote messages unless the system acts as a log server. If the system needs to act as a central log server, add the following lines to /etc/rsyslog.conf to enable reception of messages over TCP: $ModLoad imtcp.so $InputTCPServerRun 514 If the system needs to act as a log server, this ensures that it can receive messages over a reliable TCP connection. Parkinson's Disease Chastity
NIST SP800-53 AU-9
lowCCE-TODOEnable rsyslog to Accept Messages via UDP, if Acting As Log ServerThe rsyslog daemon should not accept remote messages unless the system acts as a log server. If the system needs to act as a central log server, add the following lines to /etc/rsyslog.conf to enable reception of messages over UDP: $ModLoad imudp.so $InputUDPServerRun 514 Many devices, such as switches, routers, and other Unix-like systems, may only support the traditional syslog transmission over UDP. If the system must act as a log server, this enables it to receive their messages as well. Fungal Infections Chastity
NIST SP800-53 AU-9
Ensure All Logs are Rotated by logrotateEdit the file /etc/logrotate.d/syslog. Find the first line, which should look like this (wrapped for clarity): /var/log/messages /var/log/secure /var/log/maillog /var/log/spooler \ /var/log/boot.log /var/log/cron { Edit this line so that it contains a one-space-separated listing of each log file referenced in /etc/rsyslog.conf. All logs in use on a system must be rotated regularly, or the log files will consume disk space over time, eventually interfering with system operation. The file /etc/logrotate.d/syslog is the configuration file used by the logrotate program to maintain all log files written by syslog. By default, it rotates logs weekly and stores four archival copies of each log. These settings can be modified by editing /etc/logrotate.conf, but the defaults are sufficient for purposes of this guide. Note that logrotate is run nightly by the cron job /etc/cron.daily/logrotate. If particularly active logs need to be rotated more often than once a day, some other mechanism must be used.
lowCCE-27014-0Ensure Logrotate Runs PeriodicallyThe logrotate service should be enabled.Log files that are not properly rotated run the risk of growing so large that they fill up the /var/log partition. Valuable logging information could be lost if the /var/log partition becomes full. Sore Throat Kindness
NIST SP800-53 AU-9
DISA CCI 366
Configure Logwatch on the Central Log Server Is this machine the central log server? If so, edit the file /etc/logwatch/conf/logwatch.conf as shown below.
lowCCE-27197-3Configure Logwatch HostLimit Line On a central logserver, you want Logwatch to summarize all syslog entries, including those which did not originate on the logserver itself. The HostLimit setting tells Logwatch to report on all hosts, not just the one on which it is running. HostLimit = no Cuts Chastity
lowCCE-27069-4Configure Logwatch SplitHosts Line If SplitHosts is set, Logwatch will separate entries by hostname. This makes the report longer but significantly more usable. If it is not set, then Logwatch will not report which host generated a given log entry, and that information is almost always necessary SplitHosts = yes Stress Kindness
lowns Disable Logwatch on Clients if a Logserver Exists Does your site have a central logserver which has been configured to report on logs received from all systems? If so: # rm /etc/cron.daily/0logwatch If no logserver exists, it will be necessary for each machine to run Logwatch individually. Using a central logserver provides the security and reliability benefits discussed earlier, and also makes monitoring logs easier and less time-intensive for administrators. Asthma Diligence
System Accounting with auditdThe audit service provides substantial capabilities for recording system activities. By default, the service audits about SELinux AVC denials and certain types of security-relevant events such as system logins, account modifications, and authentication events performed by programs such as sudo. Under its default configuration, auditd has modest disk space requirements, and should not noticeably impact system performance. Government networks often have substantial auditing requirements and auditd can be configured to meet these requirements. Examining some example audit records demonstrates how the Linux audit system satisfies common requirements. The following example from Fedora Documentation available at http://docs.fedoraproject.org/en-US/Fedora/13/html/Security-Enhanced_Linux/sect-Security-Enhanced_Linux-Fixing_Problems-Raw_Audit_Messages.html shows the substantial amount of information captured in a two typical "raw" audit messages, followed by a breakdown of the most important fields. In this example the message is SELinux-related and reports an AVC denial (and the associated system call) that occurred when the Apache HTTP Server attempted to access the /var/www/html/file1 file (labeled with the samba_share_t type): type=AVC msg=audit(1226874073.147:96): avc: denied { getattr } for pid=2465 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file type=SYSCALL msg=audit(1226874073.147:96): arch=40000003 syscall=196 success=no exit=-13 a0=b98df198 a1=bfec85dc a2=54dff4 a3=2008171 items=0 ppid=2463 pid=2465 auid=502 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=6 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null) msg=audit(1226874073.147:96)The number in parentheses is the unformatted time stamp (Epoch time) for the event, which can be converted to standard time by using the date command. { getattr }The item in braces indicates the permission that was denied. getattr indicates the source process was trying to read the target file's status information. This occurs before reading files. This action is denied due to the file being accessed having the wrong label. Commonly seen permissions include getattr, read, and write.comm="httpd"The executable that launched the process. The full path of the executable is found in the exe= section of the system call (SYSCALL) message, which in this case, is exe="/usr/sbin/httpd". path="/var/www/html/file1"The path to the object (target) the process attempted to access. scontext="unconfined_u:system_r:httpd_t:s0"The SELinux context of the process that attempted the denied action. In this case, it is the SELinux context of the Apache HTTP Server, which is running in the httpd_t domain. tcontext="unconfined_u:object_r:samba_share_t:s0"The SELinux context of the object (target) the process attempted to access. In this case, it is the SELinux context of file1. Note: the samba_share_t type is not accessible to processes running in the httpd_t domain. From the system call (SYSCALL) message, two items are of interest: success=no: indicates whether the denial (AVC) was enforced or not. success=no indicates the system call was not successful (SELinux denied access). success=yes indicates the system call was successful - this can be seen for permissive domains or unconfined domains, such as initrc_t and kernel_t. exe="/usr/sbin/httpd": the full path to the executable that launched the process, which in this case, is exe="/usr/sbin/httpd".
mediumCCE-27058-7Enable auditd ServiceThe auditd service is an essential userspace component of the Linux Auditing System, as it is responsible for writing audit records to disk. The auditd service can be enabled with the following command: # chkconfig auditd on Ensuring the auditd service is active ensures audit records generated by the kernel can be written to disk, or that appropriate actions will be taken if other obstacles exist. Upset Stomach Patience
DISA CCI 347, 157, 172, 880, 1353, 1462, 1487, 1115, 1454, 067, 158, 831, 1190, 1312, 1263, 130, 120, 1589
mediumCCE-26785-6Enable Auditing for Processes Which Start Prior to the Audit DaemonTo ensure all processes can be audited, even those which start prior to the audit daemon, add the argument audit=1 to the kernel line in /etc/grub.conf, in the manner below: kernel /vmlinuz-version ro vga=ext root=/dev/VolGroup00/LogVol00 rhgb quiet audit=1 Each process on the system carries an "auditable" flag which indicates whether its activities can be audited. Although auditd takes care of enabling this for all processes which launch after it does, adding the kernel argument ensures it is set for every process during boot. Chapped Lips Diligence
DISA CCI 1464, 130
Configure auditd Data Retention The audit system writes data to /var/log/audit/audit.log. By default, auditd rotates 5 logs by size (6MB), retaining a maximum of 30MB of data in total, and refuses to write entries when the disk is too full. This minimizes the risk of audit data filling its partition and impacting other services. This also minimizes the risk of the audit daemon temporarily disabling the system if it cannot write audit log (which it can be configured to do). For a busy system or a system which is thoroughly auditing system activity, the default settings for data retention may be insufficient. The log file size needed will depend heavily on what types of events are being audited. First configure auditing to log all the events of interest. Then monitor the log size manually for awhile to determine what file size will allow you to keep the required data for the correct time period. Using a dedicated partition for /var/log/audit prevents the auditd logs from disrupting system functionality if they fill, and, more importantly, prevents other activity in /var from filling the partition and stopping the audit trail. (The audit logs are size-limited and therefore unlikely to grow without bound unless configured to do so.) Some machines may have requirements that no actions occur which cannot be audited. If this is the case, then auditd can be configured to halt the machine if it runs out of space. Note: Since older logs are rotated, configuring auditd this way does not prevent older logs from being rotated away before they can be viewed. If your system is configured to halt when logging cannot be performed, make sure this can never happen under normal circumstances! Ensure that /var/log/audit is on its own partition, and that this partition is larger than the maximum amount of data auditd will retain normally.
NIST SP800-53 AU-11
DISA CCI 138
mediumnsConfigure auditd Number of Logs RetainedDetermine how many log files auditd should retain when it rotates logs. Edit the file /etc/audit/auditd.conf. Add or modify the following line, substituting NUMLOGS with the correct value: num_logs = NUMLOGS Set the value to 5 for general-purpose systems. Note that values less than 2 result in no log rotation.The total storage for audit log files must be large enough to retain log information over the period required. This is a function of the maximum log file size and the number of logs retained. Stress Chastity
NIST SP800-53 AU-1(b), AU-11
mediumnsConfigure auditd Max Log File SizeDetermine the amount of audit data (in megabytes) which should be retained in each log file. Edit the file /etc/audit/auditd.conf. Add or modify the following line, substituting the correct value for STOREMB: max_log_file = STOREMB Set the value to 6 (MB) or higher for general-purpose systems. Larger values, of course, support retention of even more audit data.The total storage for audit log files must be large enough to retain log information over the period required. This is a function of the maximum log file size and the number of logs retained. Bedwetting Charity
NIST SP800-53 AU-1(b), AU-11
mediumCCE-TODOConfigure auditd max_log_file_action Upon Reaching Maximum Log Size The default action to take when the logs reach their maximum size is to rotate the log files, discarding the oldest one. To configure the action taken by auditd, add or correct the line in /etc/audit/auditd.conf: max_log_file_action = ACTION Possible values for ACTION are described in the auditd.conf man page. These include: ignoresyslogsuspendrotatekeep_logs Set the ACTION to rotate to ensure log rotation occurs. This is the default. The setting is case-insensitive. Automatically rotating logs (by setting this to rotate) minimizes the chances of the system unexpectedly running out of disk space by being overwhelmed with log data. However, for systems that must never discard log data, or which use external processes to transfer it and reclaim space, keep_logs can be employed. Dandruff Patience
NIST SP800-53 AU-1(b), AU-4, AU-11
mediumCCE-TODOConfigure auditd space_left Action on Low Disk SpaceThe auditd service can be configured to take an action when disk space starts to run low. Edit the file /etc/audit/auditd.conf. Modify the following line, substituting ACTION appropriately: space_left_action = ACTION Possible values for ACTION are described in the auditd.conf man page. These include: ignoresyslogemailexecsuspendsinglehalt Set this to email (instead of the default, which is suspend) as it is more likely to get prompt attention. Notifying administrators of an impending disk space problem may allow them to take corrective action prior to any disruption. Spina Bifida Chastity
NIST SP800-53 AU-1(b), AU-4
DISA CCI 140, 143, 1339
mediumCCE-TODOConfigure auditd admin_space_left Action on Low Disk SpaceThe auditd service can be configured to take an action when disk space is running low but prior to running out of space completely. Edit the file /etc/audit/auditd.conf. Add or modify the following line, substituting ACTION appropriately: admin_space_left_action = ACTION Possible values for ACTION are described in the auditd.conf man page. These include: ignoresyslogemailexecsuspendsinglehalt Set this value to single to cause the system to switch to single user mode for corrective action. For certain systems, the need for availability outweighs the need to log all actions, and a different setting should be determined. Administrators should be made aware of an inability to record audit records. If a separate partition or logical volume of adequate size is used, running low on space for audit records should never occur. Asthma Chastity
NIST SP800-53 AU-1(b), AU-4
DISA CCI 140, 1343
mediumCCE-TODOConfigure auditd mail_acct Action on Low Disk SpaceThe auditd service can be configured to send email to a designated account in certain situations. Add or correct the following line in /etc/audit/auditd.conf to ensure that administrators are notified via email for those situations: action_mail_acct = root Email sent to the root account is typically aliased to the administrators of the system, who can take appropriate action. Headache Kindness
NIST SP800-53 AU-1(b), AU-4
DISA CCI 139, 144
mediumnsConfigure auditd to use audispd pluginTo configure the auditd service to use the audispd plugin, set the active line in /etc/audisp/plugins.d/syslog.conf to yes. Restart the auditdservice: # service auditd restart The auditd service does not include the ability to send audit records to a centralized server for management directly. It does, however, include an audit event multiplexor plugin (audispd) to pass audit records to the local syslog server Poison Ivy Diligence
NIST SP800-53 AU-1(b), AU-3(2)
DISA CCI 136
Configure auditd Rules for Comprehensive AuditingThe auditd program can perform comprehensive monitoring of system activity. This section describes recommended configuration settings for comprehensive auditing, but a full description of the auditing system's capabilities is beyond the scope of this guide. The mailing list linux-audit@redhat.com exists to facilitate community discussion of the auditing system. The audit subsystem supports extensive collection of events, including: Tracing of arbitrary system calls (identified by name or number) on entry or exit.Filtering by PID, UID, call success, system call argument (with some limitations), etc.Monitoring of specific files for modifications to the file's contents or metadata. Auditing rules at startup are controlled by the file /etc/audit/audit.rules. Add rules to it to meet the auditing requirements for your organization. Each line in /etc/audit/audit.rules represents a series of arguments that can be passed to auditctl and can be individually tested during runtime. See documentation in /usr/share/doc/audit-VERSION and in the related man pages for more details. If copying any example audit rulesets from /usr/share/doc/audit-VERSION, be sure to comment out the lines containing arch= which are not appropriate for your system's architecture. Then review and understand the following rules, ensuring rules are activated as needed for the appropriate architecture. After reviewing all the rules, reading the following sections, and editing as needed, the new rules can be activated as follows: # service auditd restart
Records Events that Modify Date and Time InformationArbitrary changes to the system time can be used to obfuscate nefarious activities in log files, as well as to confuse network services that are highly dependent upon an accurate system time. All changes to the system time should be audited.
lowCCE-26242-8Record attempts to alter time through adjtimexOn a 32-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b32 -S adjtimex -k audit_time_rules On a 64-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b64 -S adjtimex -k audit_time_rules The -k option allows for the specification of a key in string form that can be used for better reporting capability through ausearch and aureport. Multiple system calls can be defined on the same line to save space if desired, but is not required. See an example of multiple combined syscalls: -a always,exit -F arch=b64 -S adjtimex -S settimeofday -S clock_settime -k audit_time_rules Arbitrary changes to the system time can be used to obfuscate nefarious activities in log files, as well as to confuse network services that are highly dependent upon an accurate system time (such as sshd). All changes to the system time should be audited. Black Eye Patience
DISA CCI 1487, 169
lowCCE-27203-9Record attempts to alter time through settimeofdayOn a 32-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b32 -S settimeofday -k audit_time_rules On a 64-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b64 -S settimeofday -k audit_time_rules The -k option allows for the specification of a key in string form that can be used for better reporting capability through ausearch and aureport. Multiple system calls can be defined on the same line to save space if desired, but is not required. See an example of multiple combined syscalls: -a always,exit -F arch=b64 -S adjtimex -S settimeofday -S clock_settime -k audit_time_rules Arbitrary changes to the system time can be used to obfuscate nefarious activities in log files, as well as to confuse network services that are highly dependent upon an accurate system time (such as sshd). All changes to the system time should be audited. Migraine Diligence
DISA CCI 1487, 169
lowCCE-27169-2Record Attempts to Alter Time Through stimeOn a 32-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b32 -S stime -k audit_time_rules On a 64-bit system, the "-S time" is not necessary. The -k option allows for the specification of a key in string form that can be used for better reporting capability through ausearch and aureport. Multiple system calls can be defined on the same line to save space if desired, but is not required. See an example of multiple combined syscalls: -a always,exit -F arch=b64 -S adjtimex -S settimeofday -S clock_settime -k audit_time_rules Arbitrary changes to the system time can be used to obfuscate nefarious activities in log files, as well as to confuse network services that are highly dependent upon an accurate system time (such as sshd). All changes to the system time should be audited. Motion Sickness Charity
DISA CCI 1487, 169
lowCCE-27170-0Record Attempts to Alter Time Through clock_settimeOn a 32-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b32 -S clock_settime -k audit_time_rules On a 64-bit system, add the following to /etc/audit/audit.rules: # audit_time_rules -a always,exit -F arch=b64 -S clock_settime -k audit_time_rules The -k option allows for the specification of a key in string form that can be used for better reporting capability through ausearch and aureport. Multiple system calls can be defined on the same line to save space if desired, but is not required. See an example of multiple combined syscalls: -a always,exit -F arch=b64 -S adjtimex -S settimeofday -S clock_settime -k audit_time_rules Arbitrary changes to the system time can be used to obfuscate nefarious activities in log files, as well as to confuse network services that are highly dependent upon an accurate system time (such as sshd). All changes to the system time should be audited. Dandruff Humility
DISA CCI 1487, 169
lowCCE-27172-6Record Attempts to Alter the localtime FileAdd the following to /etc/audit/audit.rules: -w /etc/localtime -p wa -k audit_time_rules The -k option allows for the specification of a key in string form that can be used for better reporting capability through ausearch and aureport and should always be used. Arbitrary changes to the system time can be used to obfuscate nefarious activities in log files, as well as to confuse network services that are highly dependent upon an accurate system time (such as sshd). All changes to the system time should be audited. Hiccups Temperance
DISA CCI 1487, 169
lowCCE-26664-3Record Events that Modify User/Group InformationAdd the following to /etc/audit/audit.rules, in order to capture events that modify account changes: # audit_account_changes -w /etc/group -p wa -k audit_account_changes -w /etc/passwd -p wa -k audit_account_changes -w /etc/gshadow -p wa -k audit_account_changes -w /etc/shadow -p wa -k audit_account_changes -w /etc/security/opasswd -p wa -k audit_account_changes In addition to auditing new user and group accounts, these watches will alert the system administrator(s) to any modifications. Any unexpected users, groups, or modifications should be investigated for legitimacy. Muscle Aches Diligence
DISA CCI 18, 1403, 1404, 1405, 1684, 1683, 1685, 1686
lowCCE-26648-6Record Events that Modify the System's Network EnvironmentAdd the following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system: # audit_network_modifications -a exit,always -F arch=ARCH -S sethostname -S setdomainname -k audit_network_modifications -w /etc/issue -p wa -k audit_network_modifications -w /etc/issue.net -p wa -k audit_network_modifications -w /etc/hosts -p wa -k audit_network_modifications -w /etc/sysconfig/network -p wa -k audit_network_modifications The network environment should not be modified by anything other than administrator action. Any change to network parameters should be audited. Muscle Cramping Patience
lowCCE-TODOSystem Audit Logs Must Have Mode 0640 or Less Permissive Change the mode of the audit log files with the following command: # chmod 0640 audit_file If users can write to audit logs, audit trails can be modified or destroyed. Chickenpox Kindness
NIST SP800-53 AC-6, AU-1(b), AU-9
DISA CCI 166
lowCCE-TODOSystem Audit Logs Must Be Owned By Root To properly set the owner of /var/log, run the command: # chown root /var/log Failure to give ownership of the audit log file(s) to root allows the designated owner, and unauthorized users, potential access to sensitive information. Bad Breath Chastity
NIST SP800-53 AC-6, AU-1(b), AU-9
DISA CCI 166
lowCCE-26657-7Record Events that Modify the System's Mandatory Access ControlsAdd the following to /etc/audit/audit.rules: -w /etc/selinux/ -p wa -k MAC-policy The system's mandatory access policy (SELinux) should not be arbitrarily changed by anything other than administrator action. All changes to MAC policy should be audited. Obesity Temperance
Record Events that Modify the System's Discretionary Access ControlsAt a minimum the audit system should collect file permission changes for all users and root. Note that the "-F arch=b32" lines should be present even on a 64 bit system. These commands identify system calls for auditing. Even if the system is 64 bit it can still execute 32 bit system calls. Additionally, these rules can be configured in a number of ways while still achieving the desired effect. An example of this is that the "-S" calls could be split up and placed on separate lines, however, this is less efficient. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S chmod -S fchmod -S fchmodat \ -F auid>=500 -F auid!=4294967295 -k perm_mod -a always,exit -F arch=b32 -S chown -S fchown -S fchownat \ -S lchown -F auid>=500 -F auid!=4294967295 -k perm_mod -a always,exit -F arch=b32 -S setxattr -S lsetxattr \ -S fsetxattr -S removexattr -S lremovexattr -S fremovexattr \ -F auid>=500 -F auid!=4294967295 -k perm_mod If your system is 64 bit then these lines should be duplicated and the arch=b32 replaced with arch=b64 as follows: -a always,exit -F arch=b64 -S chmod -S fchmod -S fchmodat \ -F auid>=500 -F auid!=4294967295 -k perm_mod -a always,exit -F arch=b64 -S chown -S fchown -S fchownat \ -S lchown -F auid>=500 -F auid!=4294967295 -k perm_mod -a always,exit -F arch=b64 -S setxattr -S lsetxattr \ -S fsetxattr -S removexattr -S lremovexattr -S fremovexattr \ -F auid>=500 -F auid!=4294967295 -k perm_mod
lowCCE-26280-8Record Events that Modify the System's Discretionary Access Controls - chmodAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S chmod -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S chmod -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. High Cholesterol Temperance
DISA CCI 126
lowCCE-27173-4Record Events that Modify the System's Discretionary Access Controls - chownAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S chown -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S chown -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Sprain Charity
DISA CCI 126
lowCCE-27174-2Record Events that Modify the System's Discretionary Access Controls - fchmodAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S fchmod -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S fchmod -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Diaper Rash Humility
DISA CCI 126
lowCCE-27175-9Record Events that Modify the System's Discretionary Access Controls - fchmodatAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S fchmodat -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S fchmodat -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Diabetes Kindness
DISA CCI 126
lowCCE-27177-5Record Events that Modify the System's Discretionary Access Controls - fchownAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S fchown -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S fchown -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Pain Chastity
DISA CCI 126
lowCCE-27178-3Record Events that Modify the System's Discretionary Access Controls - fchownatAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S fchownat -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S fchownat -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. The Common Cold Kindness
DISA CCI 126
lowCCE-27179-1Record Events that Modify the System's Discretionary Access Controls - fremovexattrAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S fremovexattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S fremovexattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Muscle Soreness Kindness
DISA CCI 126
lowCCE-27180-9Record Events that Modify the System's Discretionary Access Controls - fsetxattrAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S fsetxattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S fsetxattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Dandruff Temperance
DISA CCI 126
lowCCE-27181-7Record Events that Modify the System's Discretionary Access Controls - lchownAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S lchown -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S lchown -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Carpal Tunnel Syndrome Patience
DISA CCI 126
lowCCE-27182-5Record Events that Modify the System's Discretionary Access Controls - lremovexattrAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S lremovexattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S lremovexattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Bee Stings Patience
DISA CCI 126
lowCCE-27183-3Record Events that Modify the System's Discretionary Access Controls - lsetxattrAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S lsetxattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S lsetxattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Sore Throat Kindness
DISA CCI 126
lowCCE-27184-1Record Events that Modify the System's Discretionary Access Controls - removexattrAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S removexattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S removexattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Carpal Tunnel Syndrome Charity
DISA CCI 126
lowCCE-27185-8Record Events that Modify the System's Discretionary Access Controls - setxattrAt a minimum the audit system should collect file permission changes for all users and root. Add the following to /etc/audit/audit.rules: -a always,exit -F arch=b32 -S setxattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod If the system is 64 bit then also add the following: -a always,exit -F arch=b64 -S setxattr -F auid>=500 -F auid!=4294967295 \ -k perm_mod The changing of file permissions could indicate that a user is attempting to gain access to information that would otherwise be disallowed. Auditing DAC modifications can facilitate the identification of patterns of abuse among both authorized and unauthorized users. Decongestion Diligence
DISA CCI 126
lowCCE-26691-6Record Attempts to Alter Logon and Logout Events The audit system already collects login info for all users and root. To watch for attempted manual edits of files involved in storing logon events, add the following to /etc/audit/audit.rules: -w /var/log/faillog -p wa -k logins -w /var/log/lastlog -p wa -k logins Manual editing of these files may indicate nefarious activity, such as an attacker attempting to remove evidence of an intrusion. Chickenpox Charity
NIST SP800-53 AC-17(7), AU-1(b)
lowCCE-26610-6 Record Attempts to Alter Process and Session Initiation Information The audit system already collects process information for all users and root. To watch for attempted manual edits of files involved in storing such process information, add the following to /etc/audit/audit.rules: -w /var/run/utmp -p wa -k session -w /var/log/btmp -p wa -k session -w /var/log/wtmp -p wa -k session Manual editing of these files may indicate nefarious activity, such as an attacker attempting to remove evidence of an intrusion. Insomnia Temperance
lowCCE-26712-0Ensure auditd Collects Unauthorized Access Attempts to Files (unsuccessful)At a minimum the audit system should collect unauthorized file accesses for all users and root. Add the following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system: -a always,exit -F arch=ARCH -S creat -S open -S openat -S truncate \ -S ftruncate -F exit=-EACCES -F auid>=500 -F auid!=4294967295 -k access -a always,exit -F arch=ARCH -S creat -S open -S openat -S truncate \ -S ftruncate -F exit=-EPERM -F auid>=500 -F auid!=4294967295 -k access Unsuccessful attempts to access files could be an indicator of malicious activity on a system. Auditing these events could serve as evidence of potential system compromise. Jaundice Chastity
DISA CCI 126
lowCCE-26457-2Ensure auditd Collects Information on the Use of Privileged CommandsAt a minimum the audit system should collect the execution of privileged commands for all users and root. To find the relevant setuid programs: # find / -xdev -type f -perm -4000 -o -perm -2000 2>/dev/null Then, for each setuid program on the system, add a line of the following form to /etc/audit/audit.rules, where SETUID_PROG_PATH is the full path to each setuid program in the list: -a always,exit -F path=SETUID_PROG_PATH -F perm=x -F auid>=500 -F auid!=4294967295 -k privileged Privileged programs are subject to escalation-of-privilege attacks, which attempt to subvert their normal role of providing some necessary but limited capability. As such, motivation exists to monitor these programs for unusual activity. Eczema Charity
DISA CCI 40
lowCCE-26573-6Ensure auditd Collects Information on Exporting to Media (successful)At a minimum the audit system should collect media exportation events for all users and root. Add the following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system: -a always,exit -F arch=ARCH -S mount -F auid>=500 -F auid!=4294967295 -k export The unauthorized exportation of data to external media could result in an information leak where classified information, Privacy Act information, and intellectual property could be lost. An audit trail should be created each time a filesystem is mounted to help identify and guard against information loss. Dandruff Kindness
DISA CCI 126
lowCCE-26651-0Ensure auditd Collects File Deletion Events by UserAt a minimum the audit system should collect file deletion events for all users and root. Add the following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system: -a always,exit -F arch=ARCH -S unlink -S unlinkat -S rename -S renameat \ -F auid>=500 -F auid!=4294967295 -k delete Auditing file deletions will create an audit trail for files that are removed from the system. The audit trail could aid in system troubleshooting, as well as, detecting malicious processes that attempt to delete log files to conceal their presence. Constipation Patience
DISA CCI 126
lowCCE-26662-7Ensure auditd Collects System Administrator ActionsAt a minimum the audit system should collect administrator actions for all users and root. Add the following to /etc/audit/audit.rules: -w /etc/sudoers -p wa -k actions The actions taken by system administrators should be audited to keep a record of what was executed on the system, as well as, for accountability purposes. Jaundice Humility
DISA CCI 126
lowCCE-26611-4Ensure auditd Collects Information on Kernel Module Loading and UnloadingAdd the following to /etc/audit/audit.rules in order to capture kernel module loading and unloading events, setting ARCH to either b32 or b64 as appropriate for your system: -w /sbin/insmod -p x -k modules -w /sbin/rmmod -p x -k modules -w /sbin/modprobe -p x -k modules -a always,exit -F arch=ARCH -S init_module -S delete_module -k modules The addition/removal of kernel modules can be used to alter the behavior of the kernel and potentially introduce malicious code into kernel space. It is important to have an audit trail of modules that have been introduced into the kernel. Pink Eye Kindness
DISA CCI 126
lowCCE-26612-2Make the auditd Configuration ImmutableAdd the following to /etc/audit/audit.rules in order to make the configuration immutable: -e 2 With this setting, a reboot will be required to change any audit rules.Making the audit configuration immutable prevents accidental as well as malicious modification of the audit rules, although it may be problematic if legitimate changes are needed during system operation Poison Ivy Charity
NIST SP800-53 AC-6, AU-1(b), AU-2(a), AU-2(c), AU-2(d)
Services The best protection against vulnerable software is running less software. This section describes how to review the software which Red Hat Enterprise Linux installs on a system and disable software which is not needed. It then enumerates the software packages installed on a default RHEL6 system and provides guidance about which ones can be safely disabled. RHEL6 provides a convenient minimal install option that essentially installs the bare necessities for a functional system. When building RHEL6 servers it is highly recommended to select the minimal packages and then build up the system from there.
Obsolete ServicesThis section discusses a number of network-visible services which have historically caused problems for system security, and for which disabling or severely limiting the service has been the best available guidance for some time. As a result of this, many of these services are not installed as part of RHEL6 by default. Organizations which are running these services should switch to more secure equivalents as soon as possible. If it remains absolutely necessary to run one of these services for legacy reasons, care should be taken to restrict the service as much as possible, for instance by configuring host firewall software such as iptables to restrict access to the vulnerable service to only those remote hosts which have a known need to use it.
XinetdThe xinetd service acts as a dedicated listener for some network services (mostly, obsolete ones) and can be used to provide access controls and perform some logging. It has been largely obsoleted by other features, and it is not installed by default. The older Inetd service is not even available as part of RHEL 6.
mediumCCE-27046-2Disable xinetd Service The xinetd service can be disabled with the following command: # chkconfig xinetd off The xinetd service provides a dedicated listener service for some programs, which is no longer necessary for commonly-used network services. Disabling it ensures that these uncommon services are not running, and also prevents attacks against xinetd itself. Overall Wellness Chastity
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305
lowCCE-27005-8Uninstall xinetd PackageThe xinetd package can be uninstalled with the following command: # yum erase xinetd Removing the xinetd package decreases the risk of the xinetd service's accidental (or intentional) activation. Insomnia Kindness
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305
TelnetThe telnet protocol does not provide confidentiality or integrity for information transmitted on the network. This includes authentication information such as passwords. Organizations which use telnet should be actively working to migrate to a more secure protocol.
highCCE-26836-7Disable telnet Service The telnet service can be disabled with the following command: # chkconfig telnet off The telnet protocol uses unencrypted network communication, which means that data from the login session, including passwords and all other information transmitted during the session, can be stolen by eavesdroppers on the network. The telnet protocol is also subject to man-in-the-middle attacks. Muscle Cramping Patience
NIST SP800-53 AC-17(8), CM-7
DISA CCI 68, 1436, 197, 877, 888
highCCE-27073-6Uninstall telnet-server PackageThe telnet-server package can be uninstalled with the following command: # yum erase telnet-server Removing the telnet-server package decreases the risk of the telnet service's accidental (or intentional) activation. Influenza Charity
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305, 381
Rlogin, Rsh, and RexecThe Berkeley r-commands are legacy services which allow cleartext remote access and have an insecure trust model.
highCCE-27062-9Uninstall rsh-server PackageThe rsh-server package can be uninstalled with the following command: # yum erase rsh-server The rsh-server package provides several obsolete and insecure network services. Removing it decreases the risk of those services' accidental (or intentional) activation. Diaper Rash Kindness
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305, 381
highCCE-27208-8Disable rexec ServiceThe rexec service, which is available with the rsh-server package and runs as a service through xinetd, should be disabled. The rexec service can be disabled with the following command: # chkconfig rexec off The rexec service uses unencrypted network communications, which means that data from the login session, including passwords and all other information transmitted during the session, can be stolen by eavesdroppers on the network. Snake Bite Patience
NIST SP800-53 AC-17(8), CM-7
DISA CCI 68, 1436
highCCE-26994-4Disable rsh ServiceThe rsh service, which is available with the rsh-server package and runs as a service through xinetd, should be disabled. The rsh service can be disabled with the following command: # chkconfig rsh off The rsh service uses unencrypted network communications, which means that data from the login session, including passwords and all other information transmitted during the session, can be stolen by eavesdroppers on the network. Bloody Nose Chastity
NIST SP800-53 AC-17(8), CM-7
DISA CCI 68, 1436
highCCE-26865-6Disable rlogin ServiceThe rlogin service, which is available with the rsh-server package and runs as a service through xinetd, should be disabled. The rlogin service can be disabled with the following command: # chkconfig rlogin off The rlogin service uses unencrypted network communications, which means that data from the login session, including passwords and all other information transmitted during the session, can be stolen by eavesdroppers on the network. Arthritis Patience
NIST SP800-53 AC-17(8), CM-7
DISA CCI 1436
highCCE-TODORemove Rsh Trust FilesThe files /etc/hosts.equiv and ~/.rhosts (in each user's home directory) list remote hosts and users that are trusted by the local system when using the rshd daemon. To remove these files, run the following command to delete them from any location: # rm /etc/hosts.equiv $ rm ~/.rhosts Trust files are convenient, but when used in conjunction with the R-services, they can allow unauthenticated access to a system. Canker Sores Patience
NIST SP800-53 AC-17(8), CM-7
DISA CCI 1436
NISThe Network Information Service (NIS), also known as 'Yellow Pages' (YP), and its successor NIS+ have been made obsolete by Kerberos, LDAP, and other modern centralized authentication services. NIS should not be used because it suffers from security problems inherent in its design, such as inadequate protection of important authentication information.
mediumCCE-27079-3Uninstall ypserv PackageThe ypserv package can be uninstalled with the following command: # yum erase ypserv Removing the ypserv package decreases the risk of the accidental (or intentional) activation of NIS or NIS+ services. Poison Ivy Diligence
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305, 381
mediumCCE-26894-6Disable ypbind ServiceThe ypbind service, which allows the system to act as a client in a NIS or NIS+ domain, should be disabled. The ypbind service can be disabled with the following command: # chkconfig ypbind off Disabling the ypbind service ensures the system is not acting as a client in a NIS or NIS+ domain. Anemia Chastity
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305
TFTP Server TFTP is a lightweight version of the FTP protocol which has traditionally been used to configure networking equipment. However, TFTP provides little security, and modern versions of networking operating systems frequently support configuration via SSH or other more secure protocols. A TFTP server should be run only if no more secure method of supporting existing equipment can be found.
mediumCCE-27055-3Disable tftp ServiceThe tftp service should be disabled. The tftp service can be disabled with the following command: # chkconfig tftp off Disabling the tftp service ensures the system is not acting as a tftp server, which does not provide encryption or authentication. Dandruff Temperance
NIST SP800-53 AC-17(8), CM-7
DISA CCI 1436
mediumCCE-26946-4Uninstall tftp-server Package The tftp-server package can be removed with the following command: # yum erase tftp-server Removing the tftp-server package decreases the risk of the accidental (or intentional) activation of tftp services. Pain Humility
NIST SP800-53 AC-17(8), CM-7
DISA CCI 305
highCCE-TODOEnsure tftp Daemon Uses Secure ModeIf running the tftp service is necessary, it should be configured to change its root directory at startup. To do so, ensure /etc/xinetd.d/tftp includes -s as a command line argument, as shown in the following example (which is also the default): server_args = -s /var/lib/tftpboot Using the -s option causes the TFTP service to only serve files from the given directory. Serving files from an intentionally-specified directory reduces the risk of sharing files which should remain private. Burns Charity
NIST SP800-53 AC-17(8), CM-7
DISA CCI 366
Base ServicesThis section addresses the base services that are installed on a RHEL 6 default installation which are not covered in other sections. Some of these services listen on the network and should be treated with particular discretion. Other services are local system utilities that may or may not be extraneous. In general, system services should be disabled if not required.
lowCCE-TODODisable Automatic Bug Reporting Tool (abrtd)The Automatic Bug Reporting Tool (abrtd) daemon collects and reports crash data when an application crash is detected. Using a variety of plugins, abrtd can email crash reports to system administrators, log crash reports to files, or forward crash reports to a centralized issue tracking system such as RHTSupport. The abrtd service can be disabled with the following command: # chkconfig abrtd off Mishandling crash data could expose sensitive information about vulnerabilities in software executing on the local machine, as well as sensitive information from within a process's address space or registers. Pneumonia Humility
NIST SP800-53 AC-17(8), CM-7
DISA CCI 381
lowCCE-27061-1Disable Advanced Configuration and Power Interface (acpid)The Advanced Configuration and Power Interface Daemon (acpid) dispatches ACPI events (such as power/reset button depressed) to userspace programs. The acpid service can be disabled with the following command: # chkconfig acpid off ACPI support is highly desirable for systems in some network roles, such as laptops or desktops. For other systems, such as servers, it may permit accidental or trivially achievable denial of service situations and disabling it is appropriate. Pain Charity
NIST SP800-53 CM-7
lowCCE-TODODisable At Service (atd)The at and batch commands can be used to schedule tasks that are meant to be executed only once. This allows delayed execution in a manner similar to cron, except that it is not recurring. The daemon atd keeps track of tasks scheduled via at and batch, and executes them at the specified time. The atd service can be disabled with the following command: # chkconfig atd off The atd service could be used by an unsophisticated insider to carry out activities outside of a normal login session, which could complicate accountability. Furthermore, the need to schedule tasks with at or batch is not common. Diabetes Temperance
NIST SP800-53 CM-7
DISA CCI 381
lowCCE-TODODisable Certmonger Service (certmonger)Certmonger is a D-Bus based service that attempts to simplify interaction with certifying authorities on networks which use public-key infrastructure. It is often combined with Red Hat's IPA (Identity Policy Audit) security information management solution to aid in the management of certificates. The certmonger service can be disabled with the following command: # chkconfig certmonger off The services provided by certmonger may be essential for systems fulfilling some roles a PKI infrastructure, but its functionality is not necessary for many other use cases. Bruising Diligence
NIST SP800-53 CM-7
lowCCE-TODODisable Control Group Config (cgconfig)Control groups allow an administrator to allocate system resources (such as CPU, memory, network bandwidth, etc) among a defined group (or groups) of processes executing on a system. The cgconfig daemon starts at boot and establishes the predefined control groups. The cgconfig service can be disabled with the following command: # chkconfig cgconfig off Unless control groups are used to manage system resources, running the cgconfig service is not necessary. Sprain Patience
NIST SP800-53 CM-7
lowCCE-TODODisable Control Group Rules Engine (cgred)The cgred service moves tasks into control groups according to parameters set in the /etc/cgrules.conf configuration file. The cgred service can be disabled with the following command: # chkconfig cgred off Unless control groups are used to manage system resources, running the cgred service service is not necessary. Parkinson's Disease Temperance
NIST SP800-53 CM-7
lowCCE-26973-8Disable CPU Speed (cpuspeed)The cpuspeed service can adjust the clock speed of supported CPUs based upon the current processing load thereby conserving power and reducing heat. The cpuspeed service can be disabled with the following command: # chkconfig cpuspeed off The cpuspeed service is only necessary if adjusting the CPU clock speed provides benefit. Traditionally this has included laptops (to enhance battery life), but may also apply to server or desktop environments where conserving power is highly desirable or necessary. Fever Patience
NIST SP800-53 CM-7
lowCCE-27086-8Disable Hardware Abstraction Layer Service (haldaemon)The Hardware Abstraction Layer Daemon (haldaemon) collects and maintains information about the system's hardware configuration. This service is required on a workstation running a desktop environment, and may be necessary on any system which deals with removable media or devices. The haldaemon service can be disabled with the following command: # chkconfig haldaemon off The haldaemon provides essential functionality on systems that use removable media or devices, but can be disabled for systems that do not require these. Stress Diligence
NIST SP800-53 CM-7
lowCCE-26990-2Enable IRQ Balance (irqbalance)The irqbalance service optimizes the balance between power savings and performance through distribution of hardware interrupts across multiple processors. The irqbalance service can be enabled with the following command: # chkconfig irqbalance on In an environment with multiple processors (now common), the irqbalance service provides potential speedups for handling interrupt requests. Psoriasis Humility
NIST SP800-53 CM-7
lowCCE-26850-8Disable KDump Kernel Crash Analyzer (kdump)The kdump service provides a kernel crash dump analyzer. It uses the kexec system call to boot a secondary kernel ("capture" kernel) following a system crash, which can load information from the crashed kernel for analysis. The kdump service can be disabled with the following command: # chkconfig kdump off Unless the system is used for kernel development or testing, there is little need to run the kdump service. Chickenpox Humility
NIST SP800-53 AC-17(8), CM-7
lowCCE-27193-2Disable Software RAID Monitor (mdmonitor)The mdmonitor service is used for monitoring a software RAID (hardware RAID setups do not use this service). The mdmonitor service can be disabled with the following command: # chkconfig mdmonitor off If software RAID monitoring is not required (and it is uncommon), there is no need to run the service. Jaundice Humility
NIST SP800-53 CM-7
lowCCE-26913-4Disable D-Bus IPC Service (messagebus)D-Bus provides an IPC mechanism used by a growing list of programs, such as those used for Gnome, Bluetooth, and Avahi. Due to these dependencies, disabling D-Bus may not be practical for many systems. The messagebus service can be disabled with the following command: # chkconfig messagebus off If no services which require D-Bus are needed, then it can be disabled. As a broker for IPC between processes of different privilege levels, it could be a target for attack. However, disabling D-Bus is likely to be impractical for any system which needs to provide a graphical login session. Hives Diligence
NIST SP800-53 CM-7
lowCCE-TODODisable Network Console (netconsole)The netconsole service is responsible for loading the netconsole kernel module, which logs kernel printk messages over UDP to a syslog server. This allows debugging of problems where disk logging fails and serial consoles are impractical. The netconsole service can be disabled with the following command: # chkconfig netconsole off The netconsole service is not necessary unless there is a need to debug kernel panics, which is not common. Sunburn Skin Kindness
NIST SP800-53 AC-17(8), CM-7
DISA CCI 381
lowCCE-TODODisable ntpdate Service (ntpdate)The ntpdate service sets the local hardware clock by polling NTP servers when the system boots. It synchronizes to the NTP servers listed in /etc/ntp/step-tickers or /etc/ntp.conf and then sets the local hardware clock to the newly synchronized system time. The ntpdate service can be disabled with the following command: # chkconfig ntpdate off The ntpdate service may only be suitable for systems which are rebooted frequently enough that clock drift does not cause problems between reboots. In any event, the functionality of the ntpdate service is now available in the ntpd program and should be considered deprecated. Spina Bifida Patience
NIST SP800-53 AC-17(8), AU-8, CM-7
DISA CCI 382
lowCCE-TODODisable Odd Job Daemon (oddjobd)The oddjobd service exists to provide an interface and access control mechanism through which specified privileged tasks can run tasks for unprivileged client applications. Communication with oddjobd through the system message bus. The oddjobd service can be disabled with the following command: # chkconfig oddjobd off The oddjobd service may provide necessary functionality in some environments but it can be disabled if it is not needed. Execution of tasks by privileged programs, on behalf of unprivileged ones, has traditionally been a source of privilege escalation security issues. Pain Humility
NIST SP800-53 CM-7
DISA CCI 381
lowCCE-TODODisable Portreserve (portreserve)The portreserve service is a TCP port reservation utility that can be used to prevent portmap from binding to well known TCP ports that are required for other services. The portreserve service can be disabled with the following command: # chkconfig portreserve off The portreserve service provides helpful functionality by preventing conflicting usage of ports in the reserved port range, but it can be disabled if not needed. Sunburn Skin Charity
NIST SP800-53 AC-17(8), CM-7
lowCCE-TODOEnable Process Accounting (psacct)The process accounting service (psacct) works with programs including acct and ac to allow system administrators to view user activity, such as commands issued by users of the system. The psacct service can be enabled with the following command: # chkconfig psacct on The psacct service can provide administrators a convenient view into some user activities. However, it should be noted that the auditing system and its audit records provide more authoritative and comprehensive records. Sty Patience
NIST SP800-53 AU-12, CM-7
lowCCE-26928-2Disable Apache Qpid (qpidd)The qpidd service provides high speed, secure, guaranteed delivery services. It is an implementation of the Advanced Message Queuing Protocol. By default the qpidd service will bind to port 5672 and listen for connection attempts. The qpidd service can be disabled with the following command: # chkconfig qpidd off The qpidd service is automatically installed when the "base" package selection is selected during installation. The qpidd service listens for network connections which increases the attack surface of the system. If the system is not intended to receive AMQP traffic then the qpidd service is not needed and should be disabled or removed. Overall Wellness Kindness
NIST SP800-53 AC-17(8), CM-7
DISA CCI 382
lowCCE-TODODisable Quota Netlink (quota_nld)The quota_nld service provides notifications to users of disk space quota violations. It listens to the kernel via a netlink socket for disk quota violations and notifies the appropriate user of the violation using D-Bus or by sending a message to the terminal that the user has last accessed. The quota_nld service can be disabled with the following command: # chkconfig quota_nld off If disk quotas are enforced on the local system, then the quota_nld service likely provides useful functionality and should remain enabled. However, if disk quotas are not used or user notification of disk quota violation is not desired then there is no need to run this service. Seasonal Affective Disorder Kindness
NIST SP800-53 CM-7
lowCCE-TODODisable Network Router Discovery Daemon (rdisc)The rdisc service implements the client side of the ICMP Internet Router Discovery Protocol (IRDP), which allows discovery of routers on the local subnet. If a router is discovered then the local routing table is updated with a corresponding default route. By default this daemon is disabled. The rdisc service can be disabled with the following command: # chkconfig rdisc off General-purpose systems typically have their network and routing information configured statically by a system administrator. Workstations or some special-purpose systems often use DHCP (instead of IRDP) to retrieve dynamic network configuration information. Constipation Patience
NIST SP800-53 AC-17(8), AC-4, CM-7
DISA CCI 382
lowCCE-26846-6Disable Red Hat Network Service (rhnsd)The Red Hat Network service automatically queries Red Hat Network servers to determine whether there are any actions that should be executed, such as package updates. This only occurs if the system was registered to an RHN server or satellite and managed as such. The rhnsd service can be disabled with the following command: # chkconfig rhnsd off Although systems management and patching is extremely important to system security, management by a system outside the enterprise enclave is not desirable for some environments. However, if the system is being managed by RHN or RHN Satellite Server the rhnsd daemon can remain on. Hiccups Temperance
NIST SP800-53 AC-17(8), CM-7
DISA CCI 382
lowCCE-TODODisable Red Hat Subscription Manager Daemon (rhsmcertd)The Red Hat Subscription Manager (rhsmcertd) periodically checks for changes in the entitlement certificates for a registered system and updates it accordingly. The rhsmcertd service can be disabled with the following command: # chkconfig rhsmcertd off The rhsmcertd service can provide administrators with some additional control over which of their systems are entitled to particular subscriptions. However, for systems that are managed locally or which are not expected to require remote changes to their subscription status, it is unnecessary and can be disabled. Snake Bite Humility
NIST SP800-53 CM-7
lowCCE-TODODisable Cyrus SASL Authentication Daemon (saslauthd)The saslauthd service handles plaintext authentication requests on behalf of the SASL library. The service isolates all code requiring superuser privileges for SASL authentication into a single process, and can also be used to provide proxy authentication services to clients that do not understand SASL based authentication. The saslauthd service can be disabled with the following command: # chkconfig saslauthd off The saslauthd service provides essential functionality for performing authentication in some directory environments, such as those which use Kerberos and LDAP. For others, however, in which only local files may be consulted, it is not necessary and should be disabled. Sty Patience
NIST SP800-53 AC-17(8), CM-7
lowCCE-26853-2Disable SMART Disk Monitoring Service (smartd)SMART (Self-Monitoring, Analysis, and Reporting Technology) is a feature of hard drives that allows them to detect symptoms of disk failure and relay an appropriate warning. The smartd service can be disabled with the following command: # chkconfig smartd off SMART can help protect against denial of service due to failing hardware. Nevertheless, if it is not needed or the system's drives are not SMART-capable (such as solid state drives), it can be disabled. Fungal Infections Patience
NIST SP800-53 CM-7
lowCCE-TODODisable System Statistics Reset Service (sysstat)The sysstat service resets various I/O and CPU performance statistics to zero in order to begin counting from a fresh state at boot time. The sysstat service can be disabled with the following command: # chkconfig sysstat off By default the sysstat service merely runs a program at boot to reset the statistics, which can be retrieved using programs such as sar and sadc. These may provide useful insight into system operation, but unless used this service can be disabled. Motion Sickness Patience
NIST SP800-53 CM-7
Cron and At DaemonsThe cron and at services are used to allow commands to be executed at a later time. The cron service is required by almost all systems to perform necessary maintenance tasks, while at may or may not be required on a given system. Both daemons should be configured defensively.
mediumCCE-27070-2Enable cron ServiceThe crond service is used to execute commands at preconfigured times. It is required by almost all systems to perform necessary maintenance tasks, such as notifying root of system activity. The crond service can be enabled with the following command: # chkconfig crond on Due to its usage for maintenance and security-supporting tasks, enabling the cron daemon is essential. Insomnia Temperance
NIST SP800-53 CM-7
lownsDisable anacron ServiceThe cronie-anacron package which provides anacron functionality is installed by default. To disable anacron support, run the following commands: # yum install cronie-noanacron # yum erase cronie-anacron The anacron service provides cron functionality for systems such as laptops and workstations that may be shut down during the normal times that cron jobs are scheduled to run. On systems which do not require this additional functionality, anacron could needlessly increase the possible attack surface for an intruder. Migraine Chastity
NIST SP800-53 CM-7
lowCCE-26548-8Disable atd Service The atd service can be disabled with the following command: # chkconfig atd off Many of the periodic or delayed execution features of the at daemon can be provided through the cron daemon instead. Poison Ivy Diligence
NIST SP800-53 CM-7
Restrict at and cron to Authorized Users if Necessary The /etc/cron.allow and /etc/at.allow files contain lists of users who are allowed to use cron and at to delay execution of processes. If these files exist and if the corresponding files /etc/cron.deny and /etc/at.deny do not exist, then only users listed in the relevant allow files can run the crontab and at commands to submit jobs to be run at scheduled intervals. On many systems, only the system administrator needs the ability to schedule jobs. Note that even if a given user is not listed in cron.allow, cron jobs can still be run as that user. The cron.allow file controls only administrative access to the crontab command for scheduling and modifying cron jobs. To restrict at and cron to only authorized users: Remove the cron.deny file:# rm /etc/cron.denyEdit /etc/cron.allow, adding one line for each user allowed to use the crontab command to create cron jobs.Remove the at.deny file:# rm /etc/at.denyEdit /etc/at.allow, adding one line for each user allowed to use the at command to create at jobs.
SSH ServerThe SSH protocol is recommended for remote login and remote file transfer. SSH provides confidentiality and integrity for data exchanged between two systems, as well as server authentication, through the use of public key cryptography. The implementation included with the system is called OpenSSH, and more detailed documentation is available from its website, http://www.openssh.org. Its server program is called sshd and provided by the RPM package openssh-server.
lowCCE-27054-6Disable SSH Server If Possible (Unusual)The SSH server service, sshd, is commonly needed. However, if it can be disabled, do so. The sshd service can be disabled with the following command: # chkconfig sshd off This is unusual, as SSH is a common method for encrypted and authenticated remote access. Stinging Nettle Diligence
lowCCE-27060-3Remove SSH Server iptables Firewall exception (Unusual)By default, inbound connections to SSH's port are allowed. If the SSH server is not being used, this exception should be removed from the firewall configuration. Edit the files /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). In each file, locate and delete the line: -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT This is unusual, as SSH is a common method for encrypted and authenticated remote access. If inbound SSH connections are not expected, disallowing access to the SSH port will avoid possible exploitation of the port by an attacker. Bee Stings Diligence
Configure OpenSSH Server if NecessaryIf the system needs to act as an SSH server, then certain changes should be made to the OpenSSH daemon configuration file /etc/ssh/sshd_config. The following recommendations can be applied to this file. See the sshd_config(5) man page for more detailed information.
highCCE-27072-8Allow Only SSH Protocol 2Only SSH protocol version 2 connections should be permitted. The default setting in /etc/ssh/sshd_config is correct, and can be verified by ensuring that the following line appears: Protocol 2 SSH protocol version 1 suffers from design flaws that result in security vulnerabilities and should not be used. Influenza Chastity
NIST SP800-53 AC-17(8)
DISA CCI 776, 774, 1436
lownsLimit Users' SSH AccessBy default, the SSH configuration allows any user with an account to access the system. In order to specify the users that are allowed to login via SSH and deny all other users, add or correct the following line in the /etc/ssh/sshd_config file: DenyUsers USER1 USER2 Where USER1 and USER2 are valid user names. Specifying which accounts are allowed SSH access into the system reduces the possibility of unauthorized access to the system. Spina Bifida Charity
lowCCE-26919-1Set SSH Idle Timeout IntervalSSH allows administrators to set an idle timeout interval. After this interval has passed, the idle user will be automatically logged out. To set an idle timeout interval, edit the following line in /etc/ssh/sshd_config as follows: ClientAliveInterval interval The timeout interval is given in seconds. To have a timeout of 15 minutes, set interval to 900. If a shorter timeout has already been set for the login shell, that value will preempt any SSH setting made here. Keep in mind that some processes may stop SSH from correctly detecting that the user is idle. Causing idle users to be automatically logged out guards against compromises one system leading trivially to compromises on another. Overall Wellness Diligence
DISA CCI 879, 1133
lowCCE-26282-4Set SSH Client Alive CountTo ensure the SSH idle timeout occurs precisely when the ClientAliveCountMax is set, edit /etc/ssh/sshd_config as follows: ClientAliveCountMax 0 This ensures a user login will be terminated as soon as the ClientAliveCountMax is reached. Bloody Nose Charity
DISA CCI 879, 1133
mediumCCE-27124-7Disable SSH Support for .rhosts FilesSSH can emulate the behavior of the obsolete rsh command in allowing users to enable insecure access to their accounts via .rhosts files. To ensure this behavior is disabled, add or correct the following line in /etc/ssh/sshd_config: IgnoreRhosts yes SSH trust relationships mean a compromise on one host can allow an attacker to move trivially to other hosts. The Common Cold Charity
DISA CCI 765, 766
mediumCCE-27091-8Disable Host-Based AuthenticationSSH's cryptographic host-based authentication is more secure than .rhosts authentication, since hosts are cryptographically authenticated. However, it is not recommended that hosts unilaterally trust one another, even within an organization. To disable host-based authentication, add or correct the following line: HostbasedAuthentication no SSH trust relationships mean a compromise on one host can allow an attacker to move trivially to other hosts. Acne Kindness
DISA CCI 765, 766
mediumCCE-27100-7Disable SSH Root LoginThe root user should never be allowed to login to a system directly over a network. To disable root login via SSH, add or correct the following line: PermitRootLogin no Permitting direct root login reduces auditable information about who ran privileged commands on the system and also allows direct attack attempts on root's password. Migraine Kindness
NIST SP800-53 AC-6(2)
DISA CCI 770
highCCE-26887-0Disable SSH Access via Empty PasswordsTo explicitly disallow remote login from accounts with empty passwords, add or correct the following line: PermitEmptyPasswords no Any accounts with empty passwords should be disabled immediately, and PAM configuration should prevent users from being able to assign themselves empty passwords. Configuring this setting for the SSH daemon provides additional assurance that remote login via SSH will require a password, even in the event of misconfiguration elsewhere. Migraine Diligence
DISA CCI 765, 766
mediumCCE-27112-2Enable SSH Warning Banner To enable the warning banner and ensure it is consistent across the system, add or correct the following line in /etc/ssh/sshd_config: Banner /etc/issue Another section contains information on how to create an appropriate system-wide warning banner. The warning message reinforces policy awareness during the logon process and facilitates possible legal action against attackers. Alternatively, systems whose ownership should not be obvious should ensure usage of a banner that does not provide easy attribution. Hiccups Humility
DISA CCI 48
lowCCE-27201-3Do Not Allow SSH Environment OptionsTo ensure users are not able to present environment options to the SSH daemon, add or correct the following line in /etc/ssh/sshd_config: PermitUserEnvironment no SSH environment options potentially allow users to bypass access restriction in some configurations. Arthritis Chastity
DISA CCI 1414
mediumCCE-26555-3Use Only Approved CiphersLimit the ciphers to those algorithms which are FIPS-approved. Counter (CTR) mode is also preferred over cipher-block chaining (CBC) mode. The following line in /etc/ssh/sshd_config demonstrates use of FIPS-approved ciphers: Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc The man page sshd_config(5) contains a list of supported ciphers. Approved algorithms should impart some level of confidence in their implementation. These are also required for compliance. Overall Wellness Patience
NIST SP800-53 AC-3, AC-17(2)
DISA CCI 803, 1144, 1145, 1146
Strengthen Firewall Configuration if PossibleIf the SSH server is expected to only receive connections from the local network, then strengthen the default firewall rule for the SSH service to only accept connections from the appropriate network segment(s). Determine an appropriate network block, netwk, and network mask, mask, representing the machines on your network which will be allowed to access this SSH server. Edit the files etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). In each file, locate the line: -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT and replace it with: -A INPUT -s netwk/mask -m state --state NEW -p tcp --dport 22 -j ACCEPT
X Window SystemThe X Window System implementation included with the system is called X.org.
Disable X WindowsUnless there is a mission-critical reason for the system to run a graphical user interface, ensure X is not set to start automatically at boot and remove the X Windows software packages. There is usually no reason to run X Windows on a dedicated server machine, as it increases the system's attack surface and consumes system resources. Administrators of server systems should instead login via SSH or on the text console.
lowCCE-27119-7Disable X Windows Startup By Setting RunlevelSetting the system's runlevel to 3 will prevent automatic startup of the X server. To do so, ensure the following line in /etc/inittab features a 3 as shown: id:3:initdefault: Unnecessary services should be disabled to decrease the attack surface of the system. Chickenpox Temperance
DISA CCI 366
lowCCE-27198-1Remove the X Windows Package GroupRemoving all packages which constitute the X Window System ensures users or malicious software cannot start X. To do so, run the following command: # yum groupremove "X Window System" Unnecessary packages should not be installed to decrease the attack surface of the system. Muscle Soreness Humility
DISA CCI 366
Avahi ServerThe Avahi daemon implements the DNS Service Discovery and Multicast DNS protocols, which provide service and host discovery on a network. It allows a system to automatically identify resources on the network, such as printers or web servers. This capability is also known as mDNSresponder and is a major part of Zeroconf networking.
Disable Avahi Server if PossibleBecause the Avahi daemon service keeps an open network port, it is subject to network attacks. Disabling it can reduce the system's vulnerability to such attacks.
lowCCE-27087-6Disable Avahi Server Software The avahi-daemon service can be disabled with the following command: # chkconfig avahi-daemon off Because the Avahi daemon service keeps an open network port, it is subject to network attacks. Its functionality is convenient but is only appropriate if the local network can be trusted. Psoriasis Diligence
NIST SP800-53 CM-7
DISA CCI 366
Configure Avahi if Necessary If your system requires the Avahi daemon, its configuration can be restricted to improve security. The Avahi daemon configuration file is /etc/avahi/avahi-daemon.conf. The following security recommendations should be applied to this file: See the avahi-daemon.conf(5) man page, or documentation at http://www.avahi.org, for more detailed information about the configuration options.
lownsServe Avahi Only via Required Protocol If you are using only IPv4, edit /etc/avahi/avahi-daemon.conf and ensure the following line exists in the [server] section: use-ipv6=no Similarly, if you are using only IPv6, disable IPv4 sockets with the line: use-ipv4=no Halitosis Kindness
NIST SP800-53 CM-7
lownsCheck Avahi Responses' TTL Field To make Avahi ignore packets unless the TTL field is 255, edit /etc/avahi/avahi-daemon.conf and ensure the following line appears in the [server] section: check-response-ttl=yes This helps to ensure that only mDNS responses from the local network are processed, because the TTL field in a packet is decremented from its initial value of 255 whenever it is routed from one network to another. Although a properly-configured router or firewall should not allow mDNS packets into the local network at all, this option provides another check to ensure they are not permitted. Tooth Ache Humility
NIST SP800-53 CM-7
lownsPrevent Other Programs from Using Avahi's Port To prevent other mDNS stacks from running, edit /etc/avahi/avahi-daemon.conf and ensure the following line appears in the [server] section: disallow-other-stacks=yes This helps ensure that only Avahi is responsible for mDNS traffic coming from that port on the system. Parkinson's Disease Kindness
NIST SP800-53 CM-7
lownsDisable Avahi Publishing To prevent other mDNS stacks from running, edit /etc/avahi/avahi-daemon.conf and ensure the following line appears in the [server] section: disallow-other-stacks=yes This helps ensure that only Avahi is responsible for mDNS traffic coming from that port on the system. Dehydration Diligence
NIST SP800-53 CM-7
Restrict Information Published by Avahi If it is necessary to publish some information to the network, it should not be joined by any extraneous information, or by information supplied by a non-trusted source on the system. Prevent user applications from using Avahi to publish services by adding or correcting the following line in the [publish] section: disable-user-service-publishing=yes Implement as many of the following lines as possible, to restrict the information published by Avahi. publish-addresses=no publish-hinfo=no publish-workstation=no publish-domain=no Inspect the files in the directory /etc/avahi/services/. Unless there is an operational need to publish information about each of these services, delete the corresponding file.
NIST SP800-53 CM-7
Print SupportThe Common Unix Printing System (CUPS) service provides both local and network printing support. A system running the CUPS service can accept print jobs from other systems, process them, and send them to the appropriate printer. It also provides an interface for remote administration through a web browser. The CUPS service is installed and activated by default. The project homepage and more detailed documentation are available at http://www.cups.org.
lowCCE-26899-5Disable the CUPS Service The cups service can be disabled with the following command: # chkconfig cups off Turn off unneeded services to reduce attack surface. Chapped Lips Diligence
NIST SP800-53 CM-7
lowCCE-26884-7Enable Firewall Access to Printing ServiceIf the system must act as a network print server then iptables should be configured to allow traffic for it through. To configure iptables to allow port 631 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 631 -j ACCEPT To configure iptables to allow port 631 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 631 -j ACCEPT By default, inbound connections to the Internet Printing Protocol port are not allowed. If the print server does need to be accessed this exception should be added to the firewall configuration. Tooth Ache Diligence
NIST SP800-53 CM-7
Configure the CUPS Service if NecessaryCUPS provides the ability to easily share local printers with other machines over the network. It does this by allowing machines to share lists of available printers. Additionally, each machine that runs the CUPS service can potentially act as a print server. Whenever possible, the printer sharing and print server capabilities of CUPS should be limited or disabled. The following recommendations should demonstrate how to do just that.
lowCCE-27108-0Disable Printer Browsing Entirely if PossibleBy default, CUPS listens on the network for printer list broadcasts on UDP port 631. This functionality is called printer browsing. To disable printer browsing entirely, edit the CUPS configuration file, located at /etc/cups/cupsd.conf, to include the following: Browsing Off The CUPS print service can be configured to broadcast a list of available printers to the network. Other machines on the network, also running the CUPS print service, can be configured to listen to these broadcasts and add and configure these printers for immediate use. By disabling this browsing capability, the machine will no longer generate or receive such broadcasts. Diabetes Humility
NIST SP800-53 CM-7
lowCCE-27107-2Disable Print Server CapabilitiesTo prevent remote users from potentially connecting to and using locally configured printers, disable the CUPS print server sharing capabilities. To do so, limit how the server will listen for print jobs by removing the more generic port directive from /etc/cups/cupsd.conf: Port 631 and replacing it with the Listen directive: Listen localhost:631 This will prevent remote users from printing to locally configured printers while still allowing local users on the machine to print normally. By default, locally configured printers will not be shared over the network, but if this functionality has somehow been enabled, these recommendations will disable it again. Be sure to disable outgoing printer list broadcasts, or remote users will still be able to see the locally configured printers, even if they cannot actually print to them. To limit print serving to a particular set of users, use the Policy directive. Canker Sores Diligence
NIST SP800-53 CM-7
DHCPThe Dynamic Host Configuration Protocol (DHCP) allows systems to request and obtain an IP address and other configuration parameters from a server. In general, sites use DHCP either to allow a large pool of mobile or unknown machines to share a limited number of IP addresses, or to standardize installations by avoiding static, individual IP address configuration on hosts. It is recommended that sites avoid DHCP as much as possible. Since DHCP authentication is not well-supported, DHCP clients are open to attacks from rogue DHCP servers. Such servers can give clients incorrect information (e.g. malicious DNS server addresses) which could lead to their compromise. If a machine must act as a DHCP client or server, configure it defensively using the guidance in this section. This guide recommends configuring networking on clients by manually editing the appropriate files under /etc/sysconfig. It is also possible to use the graphical front-end programs system-config-network and system-config-network-tui, but these programs rewrite configuration files from scratch based on their defaults - destroying any manual changes - and should therefore be used with caution.
Disable DHCP Server The DHCP server dhcpd is not installed or activated by default. If the software was installed and activated, but the system does not need to act as a DHCP server, it should be disabled and removed.
mediumCCE-27074-4Disable DHCP ServiceThe dhcpd service should be disabled on any system that does not need to act as a DHCP server. The dhcpd service can be disabled with the following command: # chkconfig dhcpd off Unmanaged or unintentionally activated DHCP servers may provide faulty information to clients, interfering with the operation of a legitimate site DHCP server if there is one. Sore Throat Kindness
NIST SP800-53 CM-7
DISA CCI 366
mediumCCE-27120-5Uninstall DHCP Server PackageIf the system does not need to act as a DHCP server, the dhcp package can be uninstalled. The dhcp package can be removed with the following command: # yum erase dhcp Removing the DHCP server ensures that it cannot be easily or accidentally reactivated and disrupt network operation. Parkinson's Disease Temperance
NIST SP800-53 CM-7
DISA CCI 366
Disable DHCP ServerIf the system must act as a DHCP server, the configuration information it serves should be minimized. Also, support for other protocols and DNS-updating schemes should be explicitly disabled unless needed. The configuration file for dhcpd is called /etc/dhcpd.conf. The file begins with a number of global configuration options. The remainder of the file is divided into sections, one for each block of addresses offered by dhcpd, each of which contains configuration options specific to that address block.
lowCCE-27049-6Do Not Use Dynamic DNSTo prevent the DHCP server from receiving DNS information from clients, edit /etc/dhcpd.conf, and add or correct the following global option: ddns-update-style none; The Dynamic DNS protocol is used to remotely update the data served by a DNS server. DHCP servers can use Dynamic DNS to publish information about their clients. This setup carries security risks, and its use is not recommended. If Dynamic DNS must be used despite the risks it poses, it is critical that Dynamic DNS transactions be protected using TSIG or some other cryptographic authentication mechanism. See dhcpd.conf(5) for more information about protecting the DHCP server from passing along malicious DNS data from its clients. Rheumatoid Arthritis Humility
NIST SP800-53 CM-7
lowCCE-27106-4Deny Decline MessagesEdit /etc/dhcpd.conf and add or correct the following global option to prevent the DHCP server from responding the DHCPDECLINE messages, if possible: deny declines; The DHCPDECLINE message can be sent by a DHCP client to indicate that it does not consider the lease offered by the server to be valid. By issuing many DHCPDECLINE messages, a malicious client can exhaust the DHCP server's pool of IP addresses, causing the DHCP server to forget old address allocations. Dehydration Charity
NIST SP800-53 CM-7
lowCCE-27077-7Deny BOOTP QueriesUnless your network needs to support older BOOTP clients, disable support for the bootp protocol by adding or correcting the global option: deny bootp; The bootp option tells dhcpd to respond to BOOTP queries. If support for this simpler protocol is not needed, it should be disabled to remove attack vectors against the DHCP server. Lice Diligence
NIST SP800-53 CM-7
Minimize Served InformationEdit /etc/dhcpd.conf. Examine each address range section within the file, and ensure that the following options are not defined unless there is an operational need to provide this information via DHCP: option domain-name option domain-name-servers option nis-domain option nis-servers option ntp-servers option routers option time-offset
NIST SP800-53 CM-7
lowCCE-26898-7Configure LoggingEnsure that the following line exists in /etc/syslog.conf: daemon.* /var/log/daemon.log Configure logwatch or other log monitoring tools to summarize error conditions reported by the dhcpd process.By default, dhcpd logs notices to the daemon facility. Sending all daemon messages to a dedicated log file is part of the syslog configuration outlined in the Logging and Auditing section Muscle Cramping Temperance
NIST SP800-53
Disable DHCP Client DHCP is the default network configuration method provided by the system installer, and common on many networks. Nevertheless, manual management of IP addresses for systems implies a greater degree of management and accountability for network activity.
lowCCE-27021-5Disable DHCP Client For each interface on the system (e.g. eth0), edit /etc/sysconfig/network-scripts/ifcfg-interface and make the following changes: Correct the BOOTPROTO line to read: BOOTPROTO=static Add or correct the following lines, substituting the appropriate values based on your site's addressing scheme: NETMASK=255.255.255.0 IPADDR=192.168.1.2 GATEWAY=192.168.1.1 DHCP relies on trusting the local network. If the local network is not trusted, then it should not be used. However, the automatic configuration provided by DHCP is commonly used and the alternative, manual configuration, presents an unacceptable burden in many circumstances. Hiccups Charity
NIST SP800-53 CM-7
DISA CCI 366
Configure DHCP Client if NecessaryIf DHCP must be used, then certain configuration changes can minimize the amount of information it receives and applies from the network, and thus the amount of incorrect information a rogue DHCP server could successfully distribute. For more information on configuring dhclient, see the dhclient(8) and dhclient.conf(5) man pages.
Minimize the DHCP-Configured OptionsCreate the file /etc/dhclient.conf, and add an appropriate setting for each of the ten configuration settings which can be obtained via DHCP. For each setting, do one of the following: If the setting should not be configured remotely by the DHCP server, select an appropriate static value, and add the line: supersede setting value; If the setting should be configured remotely by the DHCP server, add the lines: request setting; require setting; For example, suppose the DHCP server should provide only the IP address itself and the subnet mask. Then the entire file should look like: supersede domain-name "example.com"; supersede domain-name-servers 192.168.1.2; supersede nis-domain ""; supersede nis-servers ""; supersede ntp-servers "ntp.example.com "; supersede routers 192.168.1.1; supersede time-offset -18000; request subnet-mask; require subnet-mask;
Network Time ProtocolThe Network Time Protocol is used to manage the system clock over a network. Computer clocks are not very accurate, so time will drift unpredictably on unmanaged systems. Central time protocols can be used both to ensure that time is consistent among a network of machines, and that their time is consistent with the outside world. Local time synchronization is recommended for all networks. If every machine on your network reliably reports the same time as every other machine, then it is much easier to correlate log messages in case of an attack. In addition, a number of cryptographic protocols (such as Kerberos) use timestamps to prevent certain types of attacks. If your network does not have synchronized time, these protocols may be unreliable or even unusable. Depending on the specifics of the network, global time accuracy may be just as important as local synchronization, or not very important at all. If your network is connected to the Internet, it is recommended that you make use of a public timeserver or one provided by your enterprise or agency, since globally accurate timestamps may be necessary if you need to investigate or respond to an attack which originated outside of your network. A typical network setup involves a small number of internal systems operating as NTP servers, and the remainder obtaining time information from those internal servers. More information on how to configure the NTP server software, including configuration of cryptographic authentication for time data, is available at http://www.ntp.org.
mediumCCE-27093-4Enable the NTP Daemon The ntpd service can be enabled with the following command: # chkconfig ntpd on Enabling the ntpd service ensures that the ntpd service will be running and that the system will synchronize its time to any servers specified. This is important whether the system is configured to be a client (and synchronize only its own clock) or it is also acting as an NTP server to other systems. Synchronizing time is essential for authentication services such as Kerberos, but it is also important for maintaining accurate logs and auditing possible security breaches. Pain Humility
NIST SP800-53 AU-8(1)
DISA CCI 160
mediumCCE-27098-3Specify a Remote NTP ServerTo specify a remote NTP server for time synchronization, edit the file /etc/ntp.conf. Add or correct the following lines, substituting the IP or hostname of a remote NTP server for ntpserver: server ntpserver This instructs the NTP software to contact that remote server to obtain time data. Synchronizing with an NTP server makes it possible to collate system logs from multiple sources or correlate computer events with real time events. Using a trusted NTP server provided by your organization is recommended. Chapped Lips Patience
NIST SP800-53 AU-8(1)
DISA CCI 160
lownsSpecify Additional Remote NTP ServersAdditional NTP servers can be specified for time synchronization in the file /etc/ntp.conf. To do so, add additional lines of the following form, substituting the IP address or hostname of a remote NTP server for ntpserver: server ntpserver Specifying additional NTP servers increases the availability of accurate time data, in the event that one of the specified servers becomes unavailable. This is typical for a system acting as an NTP server for other systems. Stress Chastity
NIST SP800-53 AU-8(1)
Mail Server Software Mail servers are used to send and receive email over the network. Mail is a very common service, and Mail Transfer Agents (MTAs) are obvious targets of network attack. Ensure that machines are not running MTAs unnecessarily, and configure needed MTAs as defensively as possible. Very few systems at any site should be configured to directly receive email over the network. Users should instead use mail clients programs to retrieve email from a central server that support protocols such as IMAP or POP3. However, it is normal for most systems to be independently capable of sending email, for instance so that cron jobs can report output to an administrator. Most MTAs, including Postfix, support a submission-only mode in which mail can be sent from the local system to a central site MTA (or directly delivered to a local account), but the system still cannot receive mail directly over a network. The alternatives program in RHEL permits selection of other mail server software (such as Sendmail), but Postfix is the default and is preferred. Postfix was coded with security in mind and can also be more effectively contained by SELinux as its modular design has resulted in separate processes performing specific actions. More information is available on its website, http://www.postfix.org.
lowCCE-26325-1Enable Postfix ServiceThe Postfix mail transfer agent is used for local mail delivery within the system. The default configuration only listens for connections to the default SMTP port (port 25) on the loopback interface (127.0.0.1). It is recommended to leave this service enabled for local mail delivery. The postfix service can be enabled with the following command: # chkconfig postfix on Local mail delivery is essential to some system maintenance and notification tasks. Diarrhea Temperance
NIST SP800-53
mediumnsUninstall Sendmail PackageSendmail is not the default mail transfer agent and is not installed by default. The sendmail package can be removed with the following command: # yum erase sendmail The sendmail software was not developed with security in mind and its design prevents it from being effectively contained by SELinux. Postfix should be used instead. Overall Wellness Patience
NIST SP800-53 CM-7
Configure SMTP For Mail ClientsThis section discusses settings for Postfix in a submission-only e-mail configuration.
mediumCCE-26780-7Disable Postfix Network Listening Edit the file /etc/postfix/main.cf to ensure that only the following inet_interfaces line appears: inet_interfaces = localhost This ensures postfix accepts mail messages (such as cron job reports) from the local system only, and not from the network, which protects it from network attack. Pink Eye Patience
NIST SP800-53 CM-7
DISA CCI 382
Configure Operating System to Protect Mail Server The guidance in this section is appropriate for any host which is operating as a site MTA, whether the mail server runs using Sendmail, Postfix, or some other software.
Use Separate Hosts for External and Internal Mail if PossibleThe mail server is a frequent target of network attack from the outside. However, since all site users receive mail, the mail server must be open to some connection from each inside users. It is strongly recommended that these functions be separated, by having an externally visible mail server which processes all incoming and outgoing mail, then forwards internal mail to a separate machine from which users can access it.
NIST SP800-53 CM-7, SC-5, SC-7
Protect the MTA Host from User AccessThe mail server contains privileged data belonging to all users and performs a vital network function. Preventing users from logging into this server is a precaution against privilege escalation or denial of service attacks which might compromise the mail service. Take steps to ensure that only system administrators are allowed shell access to the MTA host.
Restrict Remote Access to the Mail SpoolThe mail server contains privileged data belonging to all users and performs a vital network function. Preventing users from logging into this server is a precaution against privilege escalation or denial of service attacks which might compromise the mail service. Take steps to ensure that only system administrators are allowed shell access to the MTA host.
lownsConfigure iptables to Allow Access to the Mail Server To configure iptables to allow port 25 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 25 -j ACCEPT The default Iptables configuration does not allow inbound access to the SMTP service. This modification allows that access, while keeping other ports on the server in their default protected state. Arthritis Diligence
NIST SP800-53 CM-7, SC-7
lownsVerify System Logging and Log Permissions for MailEdit the file /etc/rsyslog.conf. Add or correct the following line if necessary (this is the default): mail.* -/var/log/maillog Run the following commands to ensure correct permissions on the mail log: # chown root:root /var/log/maillog # chmod 600 /var/log/maillog Ensure log will be rotated as appropriate by adding or correcting the following line if needed into the list on the first line of /etc/logrotate.d/syslog (this is the default): /var/log/maillog Lice Temperance
NIST SP800-53 AU-2, AU-9
Configure SSL Certificates for Use with SMTP AUTHIf SMTP AUTH is to be used, the use of SSL to protect credentials in transit is strongly recommended. There are also configurations for which it may be desirable to encrypt all mail in transit from one MTA to another, though such configurations are beyond the scope of this guide. In either event, the steps for creating and installing an SSL certificate are independent of the MTA in use, and are described here.
Create an SSL Certificate Change into the CA certificate directory: # cd /etc/pki/tls/certs Generate a key pair for the mail server: # openssl genrsa -out mailserverkey.pem 2048 Next, generate a certificate signing request (CSR) for the CA to sign, making sure to supply your mail server's fully qualified domain name as the Common Name: # openssl req -new -key mailserverkey.pem -out mailserver.csr Next, the mail server CSR must be signed to create the mail server certificate. You can either send the CSR to an established CA or sign it with your CA. To sign mailserver.csr using your CA: # openssl ca -in mailserver.csr -out mailservercert.pem
NIST SP800-53 CM-7, SC-12
lownsInstall the SSL CertificateCreate the PKI directory for mail certificates, if it does not already exist: # mkdir /etc/pki/tls/mail # chown root:root /etc/pki/tls/mail # chmod 755 /etc/pki/tls/mail Using removable media or some other secure transmission format, install the files generated in the previous step onto the mail server: /etc/pki/tls/mail/serverkey.pem: the private key mailserverkey.pem /etc/pki/tls/mail/servercert.pem: the certificate file mailservercert.pem Verify the ownership and permissions of these files: # chown root:root /etc/pki/tls/mail/serverkey.pem # chown root:root /etc/pki/tls/mail/servercert.pem # chmod 600 /etc/pki/tls/mail/serverkey.pem # chmod 644 /etc/pki/tls/mail/servercert.pem Verify that the CA's public certificate file has been installed as /etc/pki/tls/CA/cacert.pem, and has the correct permissions: # chown root:root /etc/pki/tls/CA/cacert.pem # chmod 644 /etc/pki/tls/CA/cacert.pem Acne Charity
NIST SP800-53 SC-12, SC-13
Configure Postfix if NecessaryPostfix stores its configuration files in the directory /etc/postfix by default. The primary configuration file is /etc/postfix/main.cf. Other files will be introduced as needed.
lownsLimit Denial of Service AttacksEdit /etc/postfix/main.cf. Add or correct the following lines: default_process_limit = 100 smtpd_client_connection_count_limit = 10 smtpd_client_connection_rate_limit = 30 queue_minfree = 20971520 header_size_limit = 51200 message_size_limit = 10485760 smtpd_recipient_limit = 100 These configuration options serve to make it more difficult for attackers to consume resources on the MTA host. The default_process_limit parameter controls how many smtpd processes can exist at a time, while smtpd_client_connection_count_limit controls the number of those which can be occupied by any one remote sender, and smtpd_client_connection_rate_limit controls the number of connections any one client can make per minute. By default, local hosts (those in mynetworks) are exempted from per-client rate limiting. The queue_minfree parameter establishes a free space threshold, in order to stop e-mail receipt before the queue filesystem is entirely full. The header_size_limit, message_size_limit, and smtpd_recipient_limit parameters place bounds on the legal sizes of messages received via SMTP. Anemia Humility
NIST SP800-53 SC-5
mediumnsConfigure SMTP Greeting BannerEdit /etc/postfix/main.cf, and add or correct the following line, substituting some other wording for the banner information if you prefer: smtpd_banner = $myhostname ESMTP The default greeting banner discloses that the listening mail process is Postfix. When remote mail senders connect to the MTA on port 25, they are greeted by an initial banner as part of the SMTP dialogue. This banner is necessary, but it frequently gives away too much information, including the MTA software which is in use, and sometimes also its version number. Remote mail senders do not need this information in order to send mail, so the banner should be changed to reveal only the hostname (which is already known and may be useful) and the word ESMTP, to indicate that the modern SMTP protocol variant is supported. Muscle Aches Kindness
NIST SP800-53 AC-22, AU-13
Control Mail RelayingPostfix's mail relay controls are implemented with the help of the smtpd recipient restrictions option, which controls the restrictions placed on the SMTP dialogue once the sender and recipient envelope addresses are known. The guidance in the following sections should be applied to all machines. If there are machines which must be allowed to relay mail, but which cannot be trusted to relay unconditionally, configure SMTP AUTH with SSL support.
lownsConfigure Trusted Networks and HostsEdit /etc/postfix/main.cf, and configure the contents of the mynetworks variable in one of the following ways: If any machine in the subnet containing the MTA may be trusted to relay messages, add or correct the following line: mynetworks_style = subnetIf only the MTA host itself is trusted to relay messages, add or correct the following line: mynetworks_style = hostIf the set of machines which can relay is more complicated, manually specify an entry for each netblock or IP address which is trusted to relay by setting the mynetworks variable directly: mynetworks = 10.0.0.0/16 , 192.168.1.0/24 , 127.0.0.1 The mynetworks variable must contain only the set of machines for which this MTA should unconditionally relay mail. This is a trust relationship - if spammers gain access to these machines, your site will effectively become an open relay. It is recommended that only machines which are managed by you or by another trusted organization be placed in mynetworks, and users of all other machines be required to use SMTP AUTH to send mail. Diabetes Humility
NIST SP800-53 IA-8, SI-8
lownsAllow Unlimited Relaying for Trusted Networks OnlyEdit /etc/postfix/main.cf, and add or correct the smtpd_recipient_restrictions definition so that it contains at least: smtpd_recipient_restrictions = ... permit_mynetworks, reject_unauth_destination, ... The full contents of smtpd_recipient_restrictions will vary by site, since this is a common place to put spam restrictions and other site-specific options. The permit_mynetworks option allows all mail to be relayed from the machines in mynetworks. Then, the reject_unauth_destination option denies all mail whose destination address is not local, preventing any other machines from relaying. These two options should always appear in this order, and should usually follow one another immediately unless SMTP AUTH is used. Burns Diligence
NIST SP800-53 SI-8
lownsRequire SMTP AUTH Before Relaying from Untrusted ClientsSMTP authentication allows remote clients to relay mail safely by requiring them to authenticate before submit- ting mail. Postfix's SMTP AUTH uses an authentication library called SASL, which is not part of Postfix itself. This section describes how to configure authentication using the Cyrus-SASL implementation. See below for a discussion of other options. To enable the use of SASL authentication, edit /etc/postfix/main.cf and add or correct the following settings: smtpd_sasl_auth_enable = yes smtpd_recipient_restrictions = ... permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, ... Then edit /usr/lib/sasl2/smtpd.conf and add or correct the following line with the correct authentication mechanism for SASL to use: pwcheck_method: saslauthd The saslauthd service can be enabled with the following command: # chkconfig saslauthd on Postfix can use either the Cyrus library or Dovecot as a source for SASL authentication. If this host is running Dovecot for some other reason, it is recommended that Dovecot's SASL support be used instead of running the Cyrus code as well. See http://www.postfix.org/SASL_README.html for instructions on implementing that configuration, which is not described in this guide. In Postfix's configuration, the directive smtpd_sasl_auth_enable tells smtpd to allow the use of the SMTP AUTH command during the SMTP dialogue, and to support that command by getting authentication information from SASL. The smtpd_recipient_restrictions directive is changed so that, if the client is not connecting from a trusted address, it is allowed to attempt authentication (permit_sasl_authenticated) in order to relay mail. The file /usr/lib/sasl2/smtpd.conf is the Cyrus-SASL configuration file. The pwcheck_method directive tells SASL how to find passwords. The simplest method, described above, is to run a separate authentication daemon, saslauthd, which is able to communicate with the system authentication system. On RHEL6, saslauthd uses PAM by default, which should work in most cases. If you have a centralized authentication system which does not work via PAM, look at the saslauthd(8) manpage to find out how to configure saslauthd for your environment. High Cholesterol Kindness
NIST SP800-53 IA-8, SI-8
lownsRequire TLS for SMTP AUTHEdit /etc/postfix/main.cf, and add or correct the following lines: smtpd_tls_CApath = /etc/pki/tls/CA smtpd_tls_CAfile = /etc/pki/tls/CA/cacert.pem smtpd_tls_cert_file = /etc/pki/tls/mail/servercert.pem smtpd_tls_key_file = /etc/pki/tls/mail/serverkey.pem smtpd_tls_security_level = may smtpd_tls_auth_only = yes These options tell Postfix to protect all SMTP AUTH transactions using TLS. The first four options describe the locations of the necessary TLS key files. The smtpd_tls_security_level directive tells smtpd to allow the STARTTLS command during the SMTP protocol exchange, but not to require it for mail senders. (Unless your site receives mail only from other trusted sites whose sysadmins can be asked to maintain a copy of your site certificate, you do not want to require TLS for all SMTP exchanges.) The smtpd_tls_auth_only directive tells smtpd to require the STARTTLS command before allowing the client to attempt to authenticate for relaying using SMTP AUTH. It may not be possible to use this directive if you must allow relaying from non-TLS-capable client software. If this is the case, simply omit that line. Overall Wellness Humility
NIST SP800-53 IA-8, SC-13
LDAPLDAP is a popular directory service, that is, a standardized way of looking up information from a central database. It is relatively simple to configure a RHEL6 machine to obtain authentication information from an LDAP server. If your network uses LDAP for authentication, be sure to configure both clients and servers securely.
Configure OpenLDAP ClientsThis guide recommends configuring OpenLDAP clients by manually editing the appropriate configuration files. RHEL6 provides an automated configuration tool called authconfig and a graphical wrapper for authconfig called system-config-authentication. However, these tools do not give sufficient flexibility over configuration. The authconfig tools do not allow you to specify locations of SSL certificate files, which is useful when trying to use SSL cleanly across several protocols. They are also overly aggressive in placing services such as netgroups and automounter maps under LDAP control, where it is safer to use LDAP only for services to which it is relevant in your environment.
mediumCCE-26690-8Configure LDAP to Use TLS For All TransactionsConfigure LDAP to enforce TLS use. First, edit the file /etc/pam_ldap.conf, and add or correct the following lines: ssl start_tls Then review the LDAP server and ensure TLS has been configured. The ssl directive specifies whether to use ssl or not. If not specified it will default to no. It should be set to start_tls rather than doing LDAP over SSL. Bloody Nose Charity
NIST SP800-53 CM-7
DISA CCI 776, 778, 1453
mediumCCE-27189-0Configure Certificate Directives for LDAP Use of TLSEnsure a copy of the site's CA certificate has been placed in the file /etc/pki/tls/CA/cacert.pem. Configure LDAP to enforce TLS use and to trust certificates signed by the site's CA. First, edit the file /etc/pam_ldap.conf, and add or correct either of the following lines: tls_cacertdir /etc/pki/tls/CA or tls_cacertfile /etc/pki/tls/CA/cacert.pem Then review the LDAP server and ensure TLS has been configured. The tls_cacertdir or tls_cacertfile directives are required when tls_checkpeer is configured (which is the default for openldap versions 2.1 and up). These directives define the path to the trust certificates signed by the site CA. Sty Charity
NIST SP800-53 CM-7
DISA CCI 776, 778, 1453
Configure OpenLDAP ServerThis section contains guidance on how to configure an OpenLDAP server to securely provide information for use in a centralized authentication service. This is not a comprehensive guide to maintaining an OpenLDAP server, but may be helpful in securing an OpenLDAP infrastructure nonetheless.
lowCCE-26858-1Uninstall openldap-servers PackageThe openldap-servers package should be removed if not in use. Is this machine the OpenLDAP server? If not, remove the package. # yum erase openldap-servers The openldap-servers RPM is not installed by default on RHEL6 machines. It is needed only by the OpenLDAP server, not by the clients which use LDAP for authentication. If the system is not intended for use as an LDAP Server it should be removed. Unnecessary packages should not be installed to decrease the attack surface of the system. Parkinson's Disease Kindness
NIST SP800-53 CM-7
DISA CCI 366
lowCCE-27191-6Configure Domain-Specific ParametersThe ldap server should be configured to use a domain specific suffix. Is this system an OpenLDAP server? If so, edit the ldap configuration file at /etc/openldap/slapd.d/cn=config/olcDatabase={*}bdb.ldif to include suffix information relevant to your domain: olcSuffix: "dc=example,dc=com " olcRootDN: "cn=Manager,dc=example,dc=com " where dc=example,dc=com is the same root you will use on the LDAP clients. These are basic LDAP configuration directives. The suffix parameter gives the root name of all information served by this LDAP server, and should be some name related to your domain. The rootdn parameter names LDAP's privileged user, who is allowed to read or write all data managed by this LDAP server. Hiccups Temperance
NIST SP800-53 AC-2, CM-7
lownsLDAP Configuration File SecurityIs this system an OpenLDAP server? If so, ensure that the configuration files are protected from unauthorized access or modification. Edit the ldap configuration file at /etc/openldap/slapd.d/cn=config/olcDatabase={*}bdb.ldif. Ensure that the configuration file has reasonable permissions: # chown root:ldap /etc/openldap/slapd.d/cn=config/olcDatabase={*}bdb.ldif # chmod 640 /etc/openldap/slapd.d/cn=config/olcDatabase={*}bdb.ldif Protect configuration files containing the hashed password the same way you would protect other files, such as /etc/shadow, which contain hashed authentication data. Sunburn Skin Chastity
NIST SP800-53 AC-6, CM-7
lownsConfigure LDAP Root PasswordIs this system an OpenLDAP server? If so, ensure that the RootDN uses a secure password. Generate a hashed password using the slappasswd utility: # slappasswd New password: Re-enter new password: This will output a hashed password string. Edit the file /etc/openldap/slapd.d/cn=config/olcDatabase={*}bdb.ldif, and add or correct the line: olcRootPW: {SSHA}hashed-password-string Be sure to select a secure password for the LDAP root user, since this user has permission to read and write all LDAP data, so a compromise of the LDAP root password will probably enable a full compromise of your site. In addition, be sure to use a reasonably strong hash function. The default hash function should be used. The default hash function is a salted SHA-1 algorithm which is FIPS 160-1 compliant. Insecure schemes such as crypt should not be used. Stress Patience
NIST SP800-53 IA-2
lowCCE-27082-7Protect LDAP Certificate FilesCreate the PKI directory for LDAP certificates if it does not already exist: # mkdir /etc/pki/tls/ldap # chown root:root /etc/pki/tls/ldap # chmod 755 /etc/pki/tls/ldap Using removable media or some other secure transmission format, install the files generated in the previous step onto the LDAP server: /etc/pki/tls/ldap/serverkey.pem: the private key ldapserverkey.pem/etc/pki/tls/ldap/servercert.pem: the certificate file ldapservercert.pem Verify the ownership and permissions of these files: # chown root:ldap /etc/pki/tls/ldap/serverkey.pem # chown root:ldap /etc/pki/tls/ldap/servercert.pem # chmod 640 /etc/pki/tls/ldap/serverkey.pem # chmod 640 /etc/pki/tls/ldap/servercert.pem Verify that the CA's public certificate file has been installed as /etc/pki/tls/CA/cacert.pem, and has the correct permissions: # mkdir /etc/pki/tls/CA # chown root:root /etc/pki/tls/CA/cacert.pem # chmod 644 /etc/pki/tls/CA/cacert.pem As a result of these steps, the LDAP server will have access to its own private certificate and the key with which that certificate is encrypted, and to the public certificate file belonging to the CA. Note that it would be possible for the key to be protected further, so that processes running as ldap could not read it. If this were done, the LDAP server process would need to be restarted manually whenever the server rebooted. Poison Sumac Humility
NIST SP800-53 AC-6, SC-11, SC-12, SC-13, SC-17
Create Top-level LDAP Structure for DomainCreate a structure for the domain itself with at least the following attributes: dn: dc=example,dc=com objectClass: dcObject objectClass: organization dc: example o: Organization Description This is a placeholder for the root of the domain's LDAP tree. Without this entry, LDAP will not be able to find any other entries for the domain.
NIST SP800-53 AC-2
Create LDAP Structures for Users and GroupsCreate LDAP structures for people (users) and for groups with at least the following attributes: dn: ou=people,dc=example,dc=com ou: people structuralObjectClass: organizationalUnit objectClass: organizationalUnit dn: ou=groups,dc=example,dc=com ou: groups structuralObjectClass: organizationalUnit objectClass: organizationalUnit Posix users and groups are the two top-level items which will be needed in order to use LDAP for authentication. These organizational units are used to identify the two categories within LDAP.
NIST SP800-53 AC-2, AC-6, SC-2
Create Unix AccountsFor each Unix user, create an LDAP entry with at least the following attributes (others may be appropriate for your site as well), using variable values appropriate to that user: dn: uid=username ,ou=people,dc=example,dc=com structuralObjectClass: inetOrgPerson objectClass: inetOrgPerson objectClass: posixAccount objectClass: shadowAccount cn: fullname sn: surname gecos: fullname gidNumber: primary-group-id homeDirectory: /home/username loginShell: /path/to/shell uid: username uidNumber: uid userPassword: {MD5}md5-hashed-password shadowMax: N In general, the LDAP schemas for users use uid to refer to the text username, and uidNumber for the numeric UID. This usage may be slightly confusing when compared to the standard Unix usage. You should not create entries for the root account or for system accounts which are unique to individual systems, but only for user accounts which are to be shared across machines, and which have authentication information (such as a password) associated with them.
NIST SP800-53 AC-2, CM-7, SC-2
Create Unix GroupsFor each Unix group, create an LDAP entry with at least the following attributes: dn: cn=groupname ,ou=groups,dc=example,dc=com cn: groupname structuralObjectClass: posixGroup objectClass: posixGroup gidNumber: gid memberUid: username1 memberUid: username2 ... memberUid: usernameN Note that each user has a primary group, identified by the gidNumber field in the user's account entry. That group must be created, but it is not necessary to list the user as a memberUid of the group. This behavior should be familiar to administrators, since it is identical to the handling of the /etc/passwd and /etc/group files. Do not create entries for the root group or for system groups, but only for groups which contain human users or which are shared across systems.
NIST SP800-53 AC-2, CM-7, SC-2
Create Groups to Administer LDAPIf a group of LDAP administrators is desired, that group must be created somewhat differently. The specification should have these attributes: dn: cn=admins ,ou=groups,dc=example,dc=com cn: admins structuralObjectClass: groupOfUniqueNames objectClass: groupOfUniqueNames uniqueMember: cn=Manager,dc=example,dc=com uniqueMember: uid=admin1-username ,ou=people,dc=example,dc=com uniqueMember: uid=admin2-username ,ou=people,dc=example,dc=com ... uniqueMember: uid=adminN-username ,ou=people,dc=example,dc=com LDAP cannot use Posix groups for its own internal authentication - it needs to compare the username specified in an authenticated bind to some internal groupOfUniqueNames. If you do not specify an LDAP administrators' group, then all LDAP management will need to be done using the LDAP root user (Manager). For reasons of auditing and error detection, it is recommended that LDAP administrators have unique identities.
NIST SP800-53 AC-2, CM-7, SC-2
lownsConfigure slapd to Protect Authentication InformationUse ldapmodify to add these entries to the database. Add or correct the following access specifications: 1. Protect the user's password by allowing the user himself or the LDAP administrators to change it, allowing the anonymous user to authenticate against it, and allowing no other access: olcAccess: to attrs=userPassword by self write by group/groupOfUniqueNames/uniqueMember="cn=admins ,ou=groups,dc=example,dc=com " write by anonymous auth by * none olcAccess: to attrs=shadowLastChange by self write by group/groupOfUniqueNames/uniqueMember="cn=admins ,ou=groups,dc=example,dc=com " write by * read 2. Allow anyone to read other information, and allow the administrators to change it: olcAccess: to * by group/groupOfUniqueNames/uniqueMember="cn=admins ,ou=groups,dc=example,dc=com " write by * read Access rules are applied in the order encountered, so more specific rules should appear first. In particular, the rule restricting access to userPassword must appear before the rule allowing access to all data. The shadowLastChange attribute is a timestamp, and is only critical if your site implements password expiration. If your site does not have an LDAP administrators group, the LDAP root user (called Manager in this guide) will be able to change data without an explicit access statement. Canker Sores Temperance
NIST SP800-53 AC-2, AC-6, CM-7, SC-2
lowCCE-27125-4Correct Permissions on LDAP Server FilesCorrect the permissions on the ldap server's files. # chown ldap:root /var/lib/ldap/* Some manual methods of inserting information into the LDAP database may leave these files with incorrect permissions. This will prevent slapd from starting correctly. Bruising Temperance
NIST SP800-53 AC-6, CM-7
lownsConfigure iptables to Allow Access to the LDAP ServerDetermine an appropriate network block representing the machines on your network which will synchronize to this server: To configure iptables to allow port 389 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -s netwk/mask -m state --state NEW -p tcp --dport 389 -j ACCEPT To configure iptables to allow port 636 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -s netwk/mask -m state --state NEW -p tcp --dport 636 -j ACCEPT The default Iptables configuration does not allow inbound access to any services. These modifications allow access to the LDAP primary (389) and encrypted-only (636) ports, while keeping all other ports on the server in their default protected state. Note: Even if the LDAP server restricts connections so that only encrypted queries are allowed, it will probably be necessary to allow traffic to the default port 389. This is true because many LDAP clients implement encryption by connecting to the primary port and issuing the STARTTLS command. Muscle Soreness Temperance
NIST SP800-53 CM-7, SC-2, SC-5, SC-7
lownsConfigure Logging for LDAPAdd or correct the following line within /etc/rsyslog.conf: local4.* Create the log file with safe permissions: # touch /var/log/ldap.log # chown root:root /var/log/ldap.log # chmod 0600 /var/log/ldap.log Edit the file /etc/logrotate.d/syslog and add the pathname /var/log/ldap.log to the space-separated list in the first line.Edit the LDAP configuration file /etc/openldap/slapd.conf and set a reasonable set of default log parameters, such as the following: loglevel stats2 OpenLDAP sends its log data to the syslog facility local4 at priority debug. By default, RHEL5 does not store this facility at all. The syslog configuration suggested here will store any output logged by slapd in the file /var/log/ldap.log, and will include that file in the standard log rotation for syslog files. By default, LDAP's logging is quite verbose. The loglevel parameter is a space-separated list of items to be logged. Specifying stats2 will reduce the log output somewhat, but this level will still produce some logging every time an LDAP query is made. (This may be appropriate, depending on your site's auditing requirements.) In order to capture only slapd startup messages, specify loglevel none. See slapd.conf(5) for detailed information about the loglevel parameter. Black Eye Temperance
NIST SP800-53 AC-6, AU-2, AU-3, AU-9
NFS and RPCThe Network File System is a popular distributed filesystem for the Unix environment, and is very widely deployed. This section discusses the circumstances under which it is possible to disable NFS and its dependencies, and then details steps which should be taken to secure NFS's configuration. This section is relevant to machines operating as NFS clients, as well as to those operating as NFS servers.
Disable All NFS Services if PossibleIf there is not a reason for the system to operate as either an NFS client or an NFS server, follow all instructions in this section to disable subsystems required by NFS.
Disable Services Used Only by NFSIf NFS is not needed, disable the NFS client daemons nfslock, rpcgssd, and rpcidmapd. All of these daemons run with elevated privileges, and many listen for network connections. If they are not needed, they should be disabled to improve system security posture.
lowCCE-27104-9Disable Network File System Lock Service (nfslock)The Network File System Lock (nfslock) service starts the required remote procedure call (RPC) processes which allow clients to lock files on the server. If the local machine is not configured to mount NFS filesystems then this service should be disabled. The nfslock service can be disabled with the following command: # chkconfig nfslock off Sunburn Skin Chastity
lowCCE-26864-9Disable Secure RPC Client Service (rpcgssd) The rpcgssd service manages RPCSEC GSS contexts required to secure protocols that use RPC (most often Kerberos and NFS). The rpcgssd service is the client-side of RPCSEC GSS. If the system does not require secure RPC then this service should be disabled. The rpcgssd service can be disabled with the following command: # chkconfig rpcgssd off Bruising Chastity
lowCCE-26870-6Disable RPC ID Mapping Service (rpcidmapd)The rpcidmapd service is used to map user names and groups to UID and GID numbers on NFSv4 mounts. If NFS is not in use on the local system then this service should be disabled. The rpcidmapd service can be disabled with the following command: # chkconfig rpcidmapd off Headache Patience
Disable netfs if PossibleTo determine if any network filesystems handled by netfs are currently mounted on the system execute the following command: # mount -t nfs,nfs4,smbfs,cifs,ncpfs If the command did not return any output then disable netfs.
lowCCE-27137-9Disable Network File Systems (netfs)The netfs script manages the boot-time mounting of several types of networked filesystems, of which NFS and Samba are the most common. If these filesystem types are not in use, the script can be disabled, protecting the system somewhat against accidental or malicious changes to /etc/fstab and against flaws in the netfs script itself. The netfs service can be disabled with the following command: # chkconfig netfs off Gallstones Chastity
Disable RPC Bind Service if PossibleIf: NFSv3 or NFSv2 is not needed (NFSv4 implementations do not require the use of the RPC Bind Service)The site does not rely on NIS for authentication information, andThe machine does not run any other RPC-based service then disable the RPC bind service. By design, the RPC model does not require particular services to listen on fixed ports, but instead uses a daemon, rpcbind, to tell prospective clients which ports to use to contact the services they are trying to reach. This model weakens system security by introducing another privileged daemon which may be directly attacked, and is unnecessary because RPC was never adopted by enough services to risk using up all the ports on a system. Unfortunately, rpcbind is central to RPC design, so it cannot be disabled if your site is using NFSv3 or NFSv2, NIS (see Section 3.2.4 for information about NIS, which is not recommended), or any third-party or custom RPC-based program. If none of these programs are in use, however, rpcbind should be disabled to improve system security. In order to get more information about whether rpcbind may be disabled on a given host, query rpcbind using the following command: # rpcinfo -p If the only services listed are portmapper and status, it is safe to disable rpcbind. If other services are listed and your site is not running NFS or NIS, investigate these services and disable them if possible.
lowCCE-TODODisable RPC Bind Service (rpcbind)The rpcbind service is responsible for mapping RPC services to the TCP ports that they listen on. The rpcbind service also directs RPC clients to the proper port number that corresponds to the service the clients wants to communicate with. Unless RPC services are needed on the local system it is recommended to disable this service. The rpcbind service can be disabled with the following command: # chkconfig rpcbind off Muscle Cramping Chastity
Configure All Machines which Use NFSThe steps in this section are appropriate for all machines which run NFS, whether they operate as clients or as servers.
Make Each Machine a Client or a Server, not BothIf NFS must be used, it should be deployed in the simplest configuration possible to avoid maintainability problems which may lead to unnecessary security exposure. Due to the reliability and security problems caused by NFS (specially NFSv3 and NFSv2), it is not a good idea for machines which act as NFS servers to also mount filesystems via NFS. At the least, crossed mounts (the situation in which each of two servers mounts a filesystem from the other) should never be used.
Restrict Access to rpcbindWhen using NFSv2 or NFSv3 which require rpcbind, access to the rpcbind service should be restricted by using TCP Wrappers. The /etc/hosts.allow and /etc/hosts.deny files are used by TCP Wrappers to determine whether specified remote hosts are allowed to access certain services. The default RPC Bind service shipped with RHEL6 has TCP Wrappers support built in, so this specification can be used to provide some protection against network attacks on rpcbind. Note: This step protects only the RPC Bind service itself. It is still possible for attackers to guess the port numbers of NFS services and attack those services directly, even if they are denied access to rpcbind. Edit the file /etc/hosts.deny. Add or correct the line: rpcbind: ALL Edit the file /etc/hosts.allow. Add or correct the line: rpcbind: IPADDR1 , IPADDR2 , ... where each IPADDR is the IP address of a server or client with which this machine shares NFS filesystems. If the machine is an NFS server, it may be simpler to use an IP netblock specification, such as 10.3.2. (this is the TCP Wrappers syntax representing the netblock 10.3.2.0/24), or a hostname specification, such as .subdomain.example.com. The use of hostnames is not recommended.
Configure NFS Services to Use Fixed Ports (NFSv3 and NFSv2)Firewalling should be done at each host and at the border firewalls to protect the NFS daemons from remote access, since NFS servers should never be accessible from outside the organization. However, by default for NFSv3 and NFSv2, the RPC Bind service assigns each NFS service to a port dynamically at service startup time. Dynamic ports cannot be protected by port filtering firewalls such as iptables. Therefore, restrict each service to always use a given port, so that firewalling can be done effectively. Note that, because of the way RPC is implemented, it is not possible to disable the RPC Bind service even if ports are assigned statically to all RPC services. In NFSv4, the mounting and locking protocols have been incorporated into the protocol, and the server listens on the the well-known TCP port 2049. As such, NFSv4 does not need to interact with the rpcbind, lockd, and rpc.statd daemons, which can and should be disabled in a pure NFSv4 environment. The rpc.mountd daemon is still required on the NFS server to setup exports, but is not involved in any over-the-wire operations.
lowCCE-27149-4Configure lockd to use static TCP portConfigure the lockd daemon to use a static TCP port as opposed to letting the RPC Bind service dynamically assign a port. Edit the file /etc/sysconfig/nfs. Add or correct the following line: LOCKD_TCPPORT=lockd-port Where lockd-port is a port which is not used by any other service on your network. Restrict service to always use a given port, so that firewalling can be done effectively. Fever Diligence
lowCCE-27063-7Configure lockd to use static UDP portConfigure the lockd daemon to use a static UDP port as opposed to letting the RPC Bind service dynamically assign a port. Edit the file /etc/sysconfig/nfs. Add or correct the following line: LOCKD_UDPPORT=lockd-port Where lockd-port is a port which is not used by any other service on your network. Restrict service to always use a given port, so that firewalling can be done effectively. Halitosis Charity
lowCCE-26889-6Configure statd to use static portConfigure the statd daemon to use a static port as opposed to letting the RPC Bind service dynamically assign a port. Edit the file /etc/sysconfig/nfs. Add or correct the following line: STATD_PORT=statd-port Where statd-port is a port which is not used by any other service on your network. Restrict service to always use a given port, so that firewalling can be done effectively. Cold Sore Humility
lowCCE-27114-8Configure mountd to use static portConfigure the mountd daemon to use a static port as opposed to letting the RPC Bind service dynamically assign a port. Edit the file /etc/sysconfig/nfs. Add or correct the following line: MOUNTD_PORT=statd-port Where mountd-port is a port which is not used by any other service on your network. Restrict service to always use a given port, so that firewalling can be done effectively. Lice Temperance
Configure NFS ClientsThe steps in this section are appropriate for machines which operate as NFS clients.
Disable NFS Server Daemons There is no need to run the NFS server daemons nfs and rpcsvcgssd except on a small number of properly secured machines designated as NFS servers. Ensure that these daemons are turned off on clients.
lowCCE-27199-9Disable Network File System (nfs)The Network File System (NFS) service allows remote hosts to mount and interact with shared filesystems on the local machine. If the local machine is not designated as a NFS server then this service should be disabled. The nfs service can be disabled with the following command: # chkconfig nfs off Unnecessary services should be disabled to decrease the attack surface of the system. Alzheimer’s Disease Diligence
lowCCE-27122-1Disable Secure RPC Server Service (rpcsvcgssd)The rpcsvcgssd service manages RPCSEC GSS contexts required to secure protocols that use RPC (most often Kerberos and NFS). The rpcsvcgssd service is the server-side of RPCSEC GSS. If the system does not require secure RPC then this service should be disabled. The rpcsvcgssd service can be disabled with the following command: # chkconfig rpcsvcgssd off Unnecessary services should be disabled to decrease the attack surface of the system. Spina Bifida Kindness
Mount Remote Filesystems with Restrictive OptionsEdit the file /etc/fstab. For each filesystem whose type (column 3) is nfs or nfs4, add the text ,nodev,nosuid to the list of mount options in column 4. If appropriate, also add ,noexec. See the section titled "Restrict Partition Mount Options" for a description of the effects of these options. In general, execution of files mounted via NFS should be considered risky because of the possibility that an adversary could intercept the request and substitute a malicious file. Allowing setuid files to be executed from remote servers is particularly risky, both for this reason and because it requires the clients to extend root-level trust to the NFS server.
mediumCCE-27090-0Mount Remote Filesystems with nodev Add the nodev option to the fourth column of /etc/fstab for the line which controls mounting of any NFS mounts. Legitimate device files should only exist in the /dev directory. NFS mounts should not present device files to users. Diaper Rash Humility
mediumCCE-26972-0Mount Remote Filesystems with nosuid Add the nosuid option to the fourth column of /etc/fstab for the line which controls mounting of any NFS mounts. NFS mounts should not present suid binaries to users. Only vendor-supplied suid executables should be installed to their default location on the local filesystem. Alzheimer’s Disease Diligence
Configure NFS ServersThe steps in this section are appropriate for machines which operate as NFS servers.
Configure the Exports File RestrictivelyLinux's NFS implementation uses the file /etc/exports to control what filesystems and directories may be accessed via NFS. (See the exports(5) manpage for more information about the format of this file.) The syntax of the exports file is not necessarily checked fully on reload, and syntax errors can leave your NFS configuration more open than intended. Therefore, exercise caution when modifying the file. The syntax of each line in /etc/exports is: /DIR host1(opt1,opt2) host2(opt3) where /DIR is a directory or filesystem to export, hostN is an IP address, netblock, hostname, domain, or netgroup to which to export, and optN is an option.
Use Access Lists to Enforce Authorization RestrictionsWhen configuring NFS exports, ensure that each export line in /etc/exports contains a list of hosts which are allowed to access that export. If no hosts are specified on an export line, then that export is available to any remote host which requests it. All lines of the exports file should specify the hosts (or subnets, if needed) which are allowed to access the exported directory, so that unknown or remote hosts will be denied. Authorized hosts can be specified in several different formats: Name or alias that is recognized by the resolverFully qualified domain nameIP addressIP subnets in the format address/netmask or address/CIDR
Export Filesystems Read-Only if PossibleIf a filesystem is being exported so that users can view the files in a convenient fashion, but there is no need for users to edit those files, exporting the filesystem read-only removes an attack vector against the server. The default filesystem export mode is ro, so do not specify rw without a good reason.
Specify UID and GID for Anonymous ConnectionsWhen an NFS server is configured to deny remote root access, a selected UID and GID are used to handle requests from the remote root user. The UID and GID should be chosen from the system to provide the appropriate level of non-privileged access. By default, the NFS server will map remote root users to the nobody local account. Specifying the anonymous UID and GID as -1 ensures that the remote root user is mapped to a local account which has no permissions on the system. To specify the UID and GID for remote root users, edit the /etc/exports file and add anonuid=-1 and anongid=-1 to the options list for each export.
lowCCE-27138-7Use Root-Squashing on All ExportsIf a filesystem is exported using root squashing, requests from root on the client are considered to be unprivileged (mapped to a user such as nobody). This provides some mild protection against remote abuse of an NFS server. Root squashing is enabled by default, and should not be disabled. Ensure that no line in /etc/exports contains the option no_root_squash. If the NFS server allows root access to local file systems from remote hosts, this access could be used to compromise the system. Carpal Tunnel Syndrome Patience
lowCCE-27121-3Restrict NFS Clients to Privileged PortsBy default, Linux's NFS implementation requires that all client requests be made from ports less than 1024. If your organization has control over machines connected to its network, and if NFS requests are prohibited at the border firewall, this offers some protection against malicious requests from unprivileged users. Therefore, the default should not be changed. Ensure that no line in /etc/exports contains the option insecure. Allowing client requests to be made from ports higher than 1024 could allow a unprivileged user to initiate an NFS connection. If the unprivileged user account has been compromised, an attacker could gain access to data on the NFS server. Poison Ivy Charity
mediumCCE-27167-6Ensure Insecure File Locking is Not AllowedBy default the NFS server requires secure file-lock requests, which require credentials from the client in order to lock a file. Most NFS clients send credentials with file lock requests, however, there are a few clients that do not send credentials when requesting a file-lock, allowing the client to only be able to lock world-readable files. To get around this, the insecure_locks option can be used so these clients can access the desired export. This poses a security risk by potentially allowing the client access to data for which it does not have authorization. Remove any instances of the insecure_locks option from the file /etc/exports. Allowing insecure file locking could allow for sensitive data to be viewed or edited by an unauthorized user. Diabetes Patience
DISA CCI 764
DNS ServerMost organizations have an operational need to run at least one nameserver. However, there are many common attacks involving DNS server software, and this server software should be disabled on any system on which it is not needed.
Disable DNS Server DNS software should be disabled on any machine which does not need to be a nameserver. Note that the BIND DNS server software is not installed on RHEL6 by default. The remainder of this section discusses secure configuration of machines which must be nameservers.
lowCCE-26873-0Disable DNS Server The named service can be disabled with the following command: # chkconfig named off All network services involve some risk of compromise due to implementation flaws and should be disabled if possible. Diabetes Chastity
NIST SP800-53 CM-7
DISA CCI 366
lowCCE-27030-6Uninstall bind PackageTo remove the bind package, which contains the named service, run the following command: # yum erase bind If there is no need to make DNS server software available, removing it provides a safeguard against its activation. Seasonal Affective Disorder Kindness
NIST SP800-53 CM-7
DISA CCI 366
Isolate DNS from Other ServicesThis section discusses mechanisms for preventing the DNS server from interfering with other services. This is done both to protect the remainder of the network should a nameserver be compromised, and to make direct attacks on nameservers more difficult.
Run DNS Software on Dedicated ServersSince DNS is a high-risk service which must frequently be made available to the entire Internet, it is strongly recommended that no other services be offered by machines which act as organizational DNS servers.
lowCCE-26957-1Run DNS Software in a chroot JailInstall the bind-chroot package: # yum install bind-chroot Place a valid named.conf file inside the chroot jail: # cp /etc/named.conf /var/named/chroot/etc/named.conf # chown root:root /var/named/chroot/etc/named.conf # chmod 644 /var/named/chroot/etc/named.conf Create and populate an appropriate zone directory within the jail, based on the options directive. If your named.conf includes: options { directory "/path/to/DIRNAME "; ... } then copy that directory and its contents from the original zone directory: # cp -r /path/to/DIRNAME /var/named/chroot/DIRNAME Add or correct the following line within /etc/sysconfig/named: ROOTDIR=/var/named/chroot Chroot jails are not foolproof. However, they serve to make it more difficult for a compromised program to be used to attack the entire host. They do this by restricting a program's ability to traverse the directory upward, so that files outside the jail are not visible to the chrooted process. Since RHEL supports a standard mechanism for placing BIND in a chroot jail, you should take advantage of this feature. Poison Ivy Patience
NIST SP800-53 CM-7
Configure Firewalls to Protect the DNS Server By default, iptables blocks access to the ports used by the DNS server. To configure iptables to allow port 53 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p udp --dport 53 -j ACCEPT To configure iptables to allow port 53 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 53 -j ACCEPT
NIST SP800-53 CM-7
Protect DNS Data from Tampering or AttackThis section discusses DNS configuration options which make it more difficult for attackers to gain access to private DNS data or to modify DNS data.
Run Separate DNS Servers for External and Internal QueriesIs it possible to run external and internal nameservers on separate machines? If so, follow the configuration guidance in this section. On the external nameserver, edit /etc/named.conf to add or correct the following directives: options { allow-query { any; }; recursion no; ... }; zone "example.com " IN { ... }; On the internal nameserver, edit /etc/named.conf. Add or correct the following directives, where SUBNET is the numerical IP representation of your organization in the form xxx.xxx.xxx.xxx/xx: acl internal { SUBNET ; localhost; }; options { allow-query { internal; }; ... }; zone "internal.example.com " IN { ... };
Use Views to Partition External and Internal InformationIf it is not possible to run external and internal nameservers on separate physical machines, run BIND9 and simulate this feature using views. Edit /etc/named.conf. Add or correct the following directives (where SUBNET is the numerical IP representation of your organization in the form xxx.xxx.xxx.xxx/xx): acl internal { SUBNET ; localhost; }; view "internal-view" { match-clients { internal; }; zone "." IN { type hint; file "db.cache"; }; zone "internal.example.com " IN { ... }; }; view "external-view" { match-clients { any; }; recursion no; zone "example.com " IN { ... }; };
lownsDisable Zone Transfers from the NameserverIs it necessary for a secondary nameserver to receive zone data via zone transfer from the primary server? If not, follow the instructions in this section. If so, see the next section for instructions on protecting zone transfers. Add or correct the following directive within /etc/named.conf: options { allow-transfer { none; }; ... } If both the primary and secondary nameserver are under your control, or if you have only one nameserver, it may be possible to use an external configuration management mechanism to distribute zone updates. In that case, it is not necessary to allow zone transfers within BIND itself, so they should be disabled to avoid the potential for abuse. Parkinson's Disease Charity
NIST SP800-53 CM-7
lownsAuthenticate Zone TransfersIf it is necessary for a secondary nameserver to receive zone data via zone transfer from the primary server, follow the instructions here. Use dnssec-keygen to create a symmetric key file in the current directory: # cd /tmp # dnssec-keygen -a HMAC-MD5 -b 128 -n HOST dns.example.com Kdns.example.com .+aaa +iiiii This output is the name of a file containing the new key. Read the file to find the base64-encoded key string: # cat Kdns.example.com .+NNN +MMMMM .key dns.example.com IN KEY 512 3 157 base64-key-string Add the directives to /etc/named.conf on the primary server: key zone-transfer-key { algorithm hmac-md5; secret "base64-key-string "; }; zone "example.com " IN { type master; allow-transfer { key zone-transfer-key; }; ... }; Add the directives below to /etc/named.conf on the secondary nameserver: key zone-transfer-key { algorithm hmac-md5; secret "base64-key-string "; }; server IP-OF-MASTER { keys { zone-transfer-key; }; }; zone "example.com " IN { type slave; masters { IP-OF-MASTER ; }; ... }; The BIND transaction signature (TSIG) functionality allows primary and secondary nameservers to use a shared secret to verify authorization to perform zone transfers. This method is more secure than using IP-based limiting to restrict nameserver access, since IP addresses can be easily spoofed. However, if you cannot configure TSIG between your servers because, for instance, the secondary nameserver is not under your control and its administrators are unwilling to configure TSIG, you can configure an allow-transfer directive with numerical IP addresses or ACLs as a last resort. Red Eyes Charity
NIST SP800-53 CM-7
lowCCE-27105-6Disable Dynamic UpdatesIs there a mission-critical reason to enable the risky dynamic update functionality? If not, edit /etc/named.conf. For each zone specification, correct the following directive if necessary: zone "example.com " IN { allow-update { none; }; ... }; Dynamic updates allow remote servers to add, delete, or modify any entries in your zone file. Therefore, they should be considered highly risky, and disabled unless there is a very good reason for their use. If dynamic updates must be allowed, IP-based ACLs are insufficient protection, since they are easily spoofed. Instead, use TSIG keys (see the previous section for an example), and consider using the update-policy directive to restrict changes to only the precise type of change needed. Spina Bifida Temperance
FTP ServerFTP is a common method for allowing remote access to files. Like telnet, the FTP protocol is unencrypted, which means that passwords and other data transmitted during the session can be captured and that the session is vulnerable to hijacking. Therefore, running the FTP server software is not recommended. However, there are some FTP server configurations which may be appropriate for some environments, particularly those which allow only read-only anonymous access as a means of downloading data available to the public.
Disable vsftpd if Possible
lowCCE-26948-0Disable vsftpd Service The vsftpd service can be disabled with the following command: # chkconfig vsftpd off Running FTP server software provides a network-based avenue of attack, and should be disabled if not needed. Furthermore, the FTP protocol is unencrypted and creates a risk of compromising sensitive information. Motion Sickness Chastity
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DISA CCI 1436
lowCCE-26687-4Uninstall vsftpd Package The vsftpd package can be removed with the following command: # yum erase vsftpd Removing the vsftpd package decreases the risk of its accidental activation. Bedwetting Temperance
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DISA CCI 1436
Use vsftpd to Provide FTP Service if Necessary
lowCCE-27187-4Install vsftpd PackageIf this machine must operate as an FTP server, install the vsftpd package via the standard channels. # yum install vsftpd After RHEL 2.1, Red Hat switched from distributing wu-ftpd with RHEL to distributing vsftpd. For security and for consistency with future Red Hat releases, the use of vsftpd is recommended. Cold Sore Chastity
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Use vsftpd to Provide FTP Service if NecessaryThe primary vsftpd configuration file is /etc/vsftpd.conf, if that file exists, or /etc/vsftpd/vsftpd.conf if it does not.
lowCCE-27142-9Enable Logging of All FTP TransactionsAdd or correct the following configuration options within the vsftpd configuration file, located at /etc/vsftpd/vsftpd.conf: xferlog_enable=YES xferlog_std_format=NO log_ftp_protocol=YES To trace malicious activity facilitated by the FTP service, it must be configured to ensure that all commands sent to the ftp server are logged using the verbose vsftpd log format. The default vsftpd log file is /var/log/vsftpd.log. Motion Sickness Chastity
mediumCCE-27145-2Create Warning Banners for All FTP UsersEdit the vsftpd configuration file, which resides at /etc/vsftpd/vsftpd.conf by default. Add or correct the following configuration options: banner_file=/etc/issue This setting will cause the system greeting banner to be used for FTP connections as well. Diabetes Humility
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Restrict the Set of Users Allowed to Access FTPThis section describes how to disable non-anonymous (password-based) FTP logins, or, if it is not possible to do this entirely due to legacy applications, how to restrict insecure FTP login to only those users who have an identified need for this access.
lowCCE-27115-5Restrict Access to Anonymous Users if PossibleIs there a mission-critical reason for users to transfer files to/from their own accounts using FTP, rather than using a secure protocol like SCP/SFTP? If not, edit the vsftpd configuration file. Add or correct the following configuration option: local_enable=NO If non-anonymous FTP logins are necessary, follow the guidance in the remainder of this section to secure these logins as much as possible.The use of non-anonymous FTP logins is strongly discouraged. Since SSH clients and servers are widely available, and since SSH provides support for a transfer mode which resembles FTP in user interface, there is no good reason to allow password-based FTP access. Arthritis Humility
Limit Users Allowed FTP Access if NecessaryIf there is a mission-critical reason for users to access their accounts via the insecure FTP protocol, limit the set of users who are allowed this access. Edit the vsftpd configuration file. Add or correct the following configuration options: userlist_enable=YES userlist_file=/etc/vsftp.ftpusers userlist_deny=NO Edit the file /etc/vsftp.ftpusers. For each user USERNAME who should be allowed to access the system via ftp, add a line containing that user's name: USERNAME If anonymous access is also required, add the anonymous usernames to /etc/vsftp.ftpusers as well. anonymous ftp
lowCCE-27117-1Disable FTP Uploads if PossibleIs there a mission-critical reason for users to upload files via FTP? If not, edit the vsftpd configuration file to add or correct the following configuration options: write_enable=NO If FTP uploads are necessary, follow the guidance in the remainder of this section to secure these transactions as much as possible.Anonymous FTP can be a convenient way to make files available for universal download. However, it is less common to have a need to allow unauthenticated users to place files on the FTP server. If this must be done, it is necessary to ensure that files cannot be uploaded and downloaded from the same directory. Hiccups Temperance
lownsPlace the FTP Home Directory on its Own PartitionBy default, the anonymous FTP root is the home directory of the ftp user account. The df command can be used to verify that this directory is on its own partition.If there is a mission-critical reason for anonymous users to upload files, precautions must be taken to prevent these users from filling a disk used by other services. Motion Sickness Temperance
Configure Firewalls to Protect the FTP ServerBy default, iptables blocks access to the ports used by the web server. To configure iptables to allow port 21 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 21 -j ACCEPT Edit the file /etc/sysconfig/iptables-config. Ensure that the space-separated list of modules contains the FTP connection tracking module: IPTABLES_MODULES="ip_conntrack_ftp"
Web ServerThe web server is responsible for providing access to content via the HTTP protocol. Web servers represent a significant security risk because: The HTTP port is commonly probed by malicious sourcesWeb server software is very complex, and includes a long history of vulnerabilitiesThe HTTP protocol is unencrypted and vulnerable to passive monitoring The system's default web server software is Apache 2 and is provided in the RPM package httpd.
Disable Apache if PossibleIf Apache was installed and activated, but the system does not need to act as a web server, then it should be disabled and removed from the system.
lowCCE-27075-1Disable httpd Service The httpd service can be disabled with the following command: # chkconfig httpd off Running web server software provides a network-based avenue of attack, and should be disabled if not needed. Motion Sickness Diligence
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lowCCE-27133-8Uninstall httpd Package The httpd package can be removed with the following command: # yum erase httpd If there is no need to make the web server software available, removing it provides a safeguard against its activation. Spina Bifida Humility
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Install Apache if NecessaryIf httpd was not installed and activated, but the system needs to act as a web server, then it should be installed on the system. Follow these guidelines to install it defensively. The httpd package can be installed with the following command: # yum install httpd This method of installation is recommended over installing the "Web Server" package group during the system installation process. The Web Server package group includes many packages which are likely extraneous, while the command-line method installs only the required httpd package itself.
Confirm Minimal Built-in Modules InstalledThe default httpd installation minimizes the number of modules that are compiled directly into the binary (core prefork http_core mod_so). This minimizes risk by limiting the capabilities allowed by the web server. Query the set of compiled-in modules using the following command: $ httpd -l If the number of compiled-in modules is significantly larger than the aforementioned set, this guide recommends re-installing httpd with a reduced configuration. Minimizing the number of modules that are compiled into the httpd binary, reduces risk by limiting the capabilities allowed by the webserver.
Secure Apache ConfigurationThe httpd configuration file is /etc/httpd/conf/httpd.conf. Apply the recommendations in the remainder of this section to this file.
Restrict Web Server Information Leakage The ServerTokens and ServerSignature directives determine how much information the web server discloses about the configuration of the system.
lownsSet httpd ServerTokens Directive to ProdServerTokens Prod restricts information in page headers, returning only the word "Apache." Add or correct the following directive in /etc/httpd/conf/httpd.conf: ServerTokens Prod Information disclosed to clients about the configuration of the web server and system could be used to plan an attack on the given system. This information disclosure should be restricted to a minimum. Ingrown Toenails Patience
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lownsSet httpd ServerSignature Directive to OffServerSignature Off restricts httpd from displaying server version number on error pages. Add or correct the following directive in /etc/httpd/conf/httpd.conf: ServerSignature Off Information disclosed to clients about the configuration of the web server and system could be used to plan an attack on the given system. This information disclosure should be restricted to a minimum. Psoriasis Temperance
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Minimize Web Server Loadable Modules A default installation of httpd includes a plethora of dynamically shared objects (DSO) that are loaded at run-time. Unlike the aforementioned compiled-in modules, a DSO can be disabled in the configuration file by removing the corresponding LoadModule directive. Note: A DSO only provides additional functionality if associated directives are included in the httpd configuration file. It should also be noted that removing a DSO will produce errors on httpd startup if the configuration file contains directives that apply to that module. Refer to http://httpd.apache.org/docs/ for details on which directives are associated with each DSO. Following each DSO removal, the configuration can be tested with the following command to check if everything still works: # service httpd configtest The purpose of each of the modules loaded by default will now be addressed one at a time. If none of a module's directives are being used, remove it.
httpd Core Modules These modules comprise a basic subset of modules that are likely needed for base httpd functionality; ensure they are not commented out in /etc/httpd/conf/httpd.conf: LoadModule auth_basic_module modules/mod_auth_basic.so LoadModule authn_default_module modules/mod_authn_default.so LoadModule authz_host_module modules/mod_authz_host.so LoadModule authz_user_module modules/mod_authz_user.so LoadModule authz_groupfile_module modules/mod_authz_groupfile.so LoadModule authz_default_module modules/mod_authz_default.so LoadModule log_config_module modules/mod_log_config.so LoadModule logio_module modules/mod_logio.so LoadModule setenvif_module modules/mod_setenvif.so LoadModule mime_module modules/mod_mome.so LoadModule autoindex_module modules/mod_autoindex.so LoadModule negotiation_module modules/mod_negotiation.so LoadModule dir_module modules/mod_dir.so LoadModule alias_module modules/mod_alias.so Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server.
Minimize Modules for HTTP Basic Authentication The following modules are necessary if this web server will provide content that will be restricted by a password. Authentication can be performed using local plain text password files (authn_file), local DBM password files (authn_dbm) or an LDAP directory. The only module required by the web server depends on your choice of authentication. Comment out the modules you don't need from the following: LoadModule authn_file_module modules/mod_authn_file.so LoadModule authn_dbm_module modules/mod_authn_dbm.so authn_alias allows for authentication based on aliases. authn_anon allows anonymous authentication similar to that of anonymous ftp sites. authz_owner allows authorization based on file ownership. authz_dbm allows for authorization based on group membership if the web server is using DBM authentication. If the above functionality is unnecessary, comment out the related module: #LoadModule authn_alias_module modules/mod_authn_alias.so #LoadModule authn_anon_module modules/mod_authn_anon.so #LoadModule authz_owner_module modules/mod_authz_owner.so #LoadModule authz_dbm_module modules/mod_authz_dbm.so
lownsDisable HTTP Digest Authentication The auth_digest module provides encrypted authentication sessions. If this functionality is unnecessary, comment out the related module: #LoadModule auth_digest_module modules/mod_auth_digest.so Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Constipation Patience
lownsDisable HTTP mod_rewrite The mod_rewrite module is very powerful and can protect against certain classes of web attacks. However, it is also very complex and has a significant history of vulnerabilities itself. If its functionality is unnecessary, comment out the related module: #LoadModule rewrite_module modules/mod_rewrite.so Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Insomnia Chastity
lownsDisable LDAP Support The ldap module provides HTTP authentication via an LDAP directory. If its functionality is unnecessary, comment out the related modules: #LoadModule ldap_module modules/mod_ldap.so #LoadModule authnz_ldap_module modules/mod_authnz_ldap.so If LDAP is to be used, SSL encryption should be used as well. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Obesity Charity
lownsDisable Server Side Includes Server Side Includes provide a method of dynamically generating web pages through the insertion of server-side code. However, the technology is also deprecated and introduces significant security concerns. If this functionality is unnecessary, comment out the related module: #LoadModule include_module modules/mod_include.so If there is a critical need for Server Side Includes, they should be enabled with the option IncludesNoExec to prevent arbitrary code execution. Additionally, user supplied data should be encoded to prevent cross-site scripting vulnerabilities. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Carpal Tunnel Syndrome Diligence
lownsDisable MIME Magic The mime_magic module provides a second layer of MIME support that in most configurations is likely extraneous. If its functionality is unnecessary, comment out the related module: #LoadModule mime_magic_module modules/mod_mime_magic.so Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Fever Charity
lownsDisable WebDAV (Distributed Authoring and Versioning) WebDAV is an extension of the HTTP protocol that provides distributed and collaborative access to web content. If its functionality is unnecessary, comment out the related modules: #LoadModule dav_module modules/mod_dav.so #LoadModule dav_fs_module modules/mod_dav_fs.so If there is a critical need for WebDAV, extra care should be taken in its configuration. Since DAV access allows remote clients to manipulate server files, any location on the server that is DAV enabled should be protected by access controls. Minimizing the number of loadable modules available to the web server, reduces risk by limiting the capabilities allowed by the web server. Pink Eye Patience
lownsDisable Server Activity Status The status module provides real-time access to statistics on the internal operation of the web server. This may constitute an unnecessary information leak and should be disabled unless necessary. To do so, comment out the related module: #LoadModule status_module modules/mod_status.so If there is a critical need for this module, ensure that access to the status page is properly restricted to a limited set of hosts in the status handler configuration. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Hangover Temperance
lownsDisable Web Server Configuration Display The info module creates a web page illustrating the configuration of the web server. This can create an unnecessary security leak and should be disabled. If its functionality is unnecessary, comment out the module: #LoadModule info_module modules/mod_info.so If there is a critical need for this module, use the Location directive to provide an access control list to restrict access to the information. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Poison Oak Charity
lownsDisable URL Correction on Misspelled Entries The speling module attempts to find a document match by allowing one misspelling in an otherwise failed request. If this functionality is unnecessary, comment out the module: #LoadModule speling_module modules/mod_speling.so This functionality weakens server security by making site enumeration easier. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Chapped Lips Kindness
lownsDisable Proxy Support The proxy module provides proxying support, allowing httpd to forward requests and serve as a gateway for other servers. If its functionality is unnecessary, comment out the module: #LoadModule proxy_module modules/mod_proxy.so If proxy support is needed, load mod_proxy and the appropriate proxy protocol handler module (one of mod_proxy_http, mod_proxy_ftp, or mod_proxy_connect). Additionally, make certain that a server is secure before enabling proxying, as open proxy servers are a security risk. mod_proxy_balancer enables load balancing, but requires that mod status be enabled. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. The Common Cold Patience
lownsDisable Cache Support The cache module allows httpd to cache data, optimizing access to frequently accessed content. However, it introduces potential security flaws such as the possibility of circumventing Allow and Deny directives. If this functionality is unnecessary, comment out the module: #LoadModule cache_module modules/mod_cache.so If caching is required, it should not be enabled for any limited-access content. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Acne Kindness
lownsDisable CGI Support The cgi module allows HTML to interact with the CGI web programming language. If this functionality is unnecessary, comment out the module: #LoadModule cgi_module modules/mod_cgi.so If the web server requires the use of CGI, enable mod_cgi. Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server. Stomach Ache Patience
Minimize Various Optional Components The following modules perform very specific tasks, sometimes providing access to just a few additional directives. If such functionality is not required (or if you are not using these directives), comment out the associated module: External filtering (response passed through external program prior to client delivery) #LoadModule ext_filter_module modules/mod_ext_filter.soUser-specified Cache Control and Expiration #LoadModule expires_module modules/mod_expires.soCompression Output Filter (provides content compression prior to client delivery) #LoadModule deflate_module modules/mod_deflate.soHTTP Response/Request Header Customization #LoadModule headers_module modules/mod_headers.soUser activity monitoring via cookies #LoadModule usertrack_module modules/mod_usertrack.soDynamically configured mass virtual hosting #LoadModule vhost_alias_module modules/mod_vhost_alias.so Minimizing the number of loadable modules available to the web server reduces risk by limiting the capabilities allowed by the web server.
Minimize Configuration Files Included The Include directive directs httpd to load supplementary configuration files from a provided path. The default configuration loads all files that end in .conf from the /etc/httpd/conf.d directory. To restrict excess configuration, the following line should be commented out and replaced with Include directives that only reference required configuration files: #Include conf.d/*.conf If the above change was made, ensure that the SSL encryption remains loaded by explicitly including the corresponding configuration file: Include conf.d/ssl.conf If PHP is necessary, a similar alteration must be made: Include conf.d/php.conf Explicitly listing the configuration files to be loaded during web server start-up avoids the possibility of unwanted or malicious configuration files to be automatically included as part of the server's running configuration.
Directory Restrictions The Directory tags in the web server configuration file allow finer grained access control for a specified directory. All web directories should be configured on a case-by-case basis, allowing access only where needed.
lownsRestrict Root Directory The httpd root directory should always have the most restrictive configuration enabled. <Directory / > Options None AllowOverride None Order allow,deny </Directory> The Web Server's root directory content should be protected from unauthorized access by web clients. Decongestion Patience
lownsRestrict Web Directory The default configuration for the web (/var/www/html) Directory allows directory indexing (Indexes) and the following of symbolic links (FollowSymLinks). Neither of these is recommended. The /var/www/html directory hierarchy should not be viewable via the web, and symlinks should only be followed if the owner of the symlink also owns the linked file. Ensure that this policy is adhered to by altering the related section of the configuration: <Directory "/var/www/html"> # ... Options SymLinksIfOwnerMatch # ... </Directory> Access to the web server's directory hierarchy could allow access to unauthorized files by web clients. Following symbolic links could also allow such access. Ingrown Toenails Kindness
lownsRestrict Other Critical Directories All accessible web directories should be configured with similarly restrictive settings. The Options directive should be limited to necessary functionality and the AllowOverride directive should be used only if needed. The Order and Deny access control tags should be used to deny access by default, allowing access only where necessary. Directories accessible from a web client should be configured with the least amount of access possible in order to avoid unauthorized access to restricted content or server information. The Common Cold Temperance
lownsLimit Available Methods Web server methods are defined in section 9 of RFC 2616 (http://www.ietf.org/rfc/rfc2616.txt). If a web server does not require the implementation of all available methods, they should be disabled. Note: GET and POST are the most common methods. A majority of the others are limited to the WebDAV protocol. <Directory /var/www/html> # ... # Only allow specific methods (this command is case-sensitive!) <LimitExcept GET POST> Order allow,deny </LimitExcept> # ... </Directory> Minimizing the number of available methods to the web client reduces risk by limiting the capabilities allowed by the web server. Bad Breath Diligence
Use Appropriate Modules to Improve httpd's Security Among the modules available for httpd are several whose use may improve the security of the web server installation. This section recommends and discusses the deployment of security-relevant modules.
Deploy mod_ssl Because HTTP is a plain text protocol, all traffic is susceptible to passive monitoring. If there is a need for confidentiality, SSL should be configured and enabled to encrypt content. Note: mod_nss is a FIPS 140-2 certified alternative to mod_ssl. The modules share a considerable amount of code and should be nearly identical in functionality. If FIPS 140-2 validation is required, then mod_nss should be used. If it provides some feature or its greater compatibility is required, then mod_ssl should be used.
lownsInstall mod_ssl Install the mod_ssl module: # yum install mod_ssl mod_ssl provides encryption capabilities for the httpd Web server. Unencrypted content is transmitted in plain text which could be passively monitored and accessed by unauthorized parties. Diabetes Patience
Deploy mod_security The security module provides an application level firewall for httpd. Following its installation with the base ruleset, specific configuration advice can be found at http://www.modsecurity.org/ to design a policy that best matches the security needs of the web applications. Usage of mod_security is highly recommended for some environments, but it should be noted this module does not ship with Red Hat Enterprise Linux itself, and instead is provided via Extra Packages for Enterprise Linux (EPEL). For more information on EPEL please refer to http://fedoraproject.org/wiki/EPEL.
lownsInstall mod_security Install the security module: # yum install mod_security mod_security provides an additional level of protection for the web server by enabling the administrator to implement content access policies and filters at the application layer. Ingrown Toenails Kindness
Use Denial-of-Service Protection Modules Denial-of-service attacks are difficult to detect and prevent while maintaining acceptable access to authorized users. However, some traffic-shaping modules can be used to address the problem. Well-known DoS protection modules include: mod_cband mod_bwshare mod_limitipconn mod_evasive Denial-of-service prevention should be implemented for a web server if such a threat exists. However, specific configuration details are very dependent on the environment and often best left at the discretion of the administrator.
Configure PHP Securely PHP is a widely-used and often misconfigured server-side scripting language. It should be used with caution, but configured appropriately when needed. Review /etc/php.ini and make the following changes if possible: # Do not expose PHP error messages to external users display_errors = Off # Enable safe mode safe_mode = On # Only allow access to executables in isolated directory safe_mode_exec_dir = php-required-executables-path # Limit external access to PHP environment safe_mode_allowed_env_vars = PHP_ # Restrict PHP information leakage expose_php = Off # Log all errors log_errors = On # Do not register globals for input data register_globals = Off # Minimize allowable PHP post size post_max_size = 1K # Ensure PHP redirects appropriately cgi.force_redirect = 0 # Disallow uploading unless necessary file_uploads = Off # Disallow treatment of file requests as fopen calls allow_url_fopen = Off # Enable SQL safe mode sql.safe_mode = On
Configure Operating System to Protect Web Server The following configuration steps should be taken on the machine which hosts the web server, in order to provide as safe an environment as possible for the web server.
Restrict File and Directory Access Minimize access to critical httpd files and directories.
lowCCE-27150-2Set Permissions on the /var/log/httpd/ Directory Ensure that the permissions on the web server log directory is set to 700: # chmod 700 /var/log/httpd/ This is its default setting. Access to the web server's log files may allow an unauthorized user or attacker to access information about the web server or alter the server's log files. Tooth Ache Kindness
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lownsSet Permissions on the /etc/httpd/conf/ Directory Set permissions on the web server configuration directory to 750: # chmod 750 /etc/httpd/conf/ Access to the web server's configuration files may allow an unauthorized user or attacker to access information about the web server or alter the server's configuration files. Diabetes Charity
lownsSet Permissions on All Configuration Files Inside /etc/httpd/conf/ Set permissions on the web server configuration files to 640: # chmod 640 /etc/httpd/conf/* Access to the web server's configuration files may allow an unauthorized user or attacker to access information about the web server or to alter the server's configuration files. Asthma Chastity
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Configure iptables to Allow Access to the Web Server By default, iptables blocks access to the ports used by the web server. To configure iptables to allow port 80 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT To configure iptables to allow port 443 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
Run httpd in a chroot Jail if Practical Running httpd inside a chroot jail is designed to isolate the web server process to a small section of the filesystem, limiting the damage if it is compromised. Versions of Apache greater than 2.2.10 (such as the one included with RHEL 6) provide the ChrootDir directive. To run Apache inside a chroot jail in /chroot/apache, add the following line to /etc/httpd/conf/httpd.conf: ChrootDir /chroot/apache This necessitates placing all files required by httpd inside /chroot/apache , including httpd's binaries, modules, configuration files, and served web pages. The details of this configuration are beyond the scope of this guide. This may also require additional SELinux configuration.
IMAP and POP3 ServerDovecot provides IMAP and POP3 services. It is not installed by default. The project page at http://www.dovecot.org contains more detailed information about Dovecot configuration.
Disable DovecotIf the system does not need to operate as an IMAP or POP3 server, the dovecot software should be disabled and removed.
lowCCE-26922-5Disable Dovecot Service The dovecot service can be disabled with the following command: # chkconfig dovecot off Running an IMAP or POP3 server provides a network-based avenue of attack, and should be disabled if not needed. Anemia Diligence
lowCCE-27039-7Uninstall dovecot PackageThe dovecot package can be uninstalled with the following command: # yum erase dovecot If there is no need to make the Dovecot software available, removing it provides a safeguard against its activation. Cuts Patience
Configure Dovecot if NecessaryIf the system will operate as an IMAP or POP3 server, the dovecot software should be configured securely by following the recommendations below.
lowCCE-27097-5Support Only the Necessary ProtocolsDovecot supports the IMAP and POP3 protocols, as well as SSL-protected versions of those protocols. Configure the Dovecot server to support only the protocols needed by your site. Edit /etc/dovecot/dovecot.conf. Add or correct the following lines, replacing PROTOCOL with only the subset of protocols (imap, imaps, pop3, pop3s) required: protocols = PROTOCOL If possible, require SSL protection for all transactions. The SSL protocol variants listen on alternate ports (995 instead of 110 for pop3s, and 993 instead of 143 for imaps), and require SSL-aware clients. An alternate approach is to listen on the standard port and require the client to use the STARTTLS command before authenticating. Configuring Dovecot to only support the protocols the protocols needed by your site reduces the risk of an attacker using one of the unused protocols to base an attack. Sprain Diligence
Enable SSL SupportSSL should be used to encrypt network traffic between the Dovecot server and its clients. Users must authenticate to the Dovecot server in order to read their mail, and passwords should never be transmitted in clear text. In addition, protecting mail as it is downloaded is a privacy measure, and clients may use SSL certificates to authenticate the server, preventing another system from impersonating the server.
lownsEnable the SSL flag in /etc/dovecot.confTo allow clients to make encrypted connections the ssl flag in Dovecot's configuration file needs to be set to yes. Edit /etc/dovecot/conf.d/10-ssl.conf and add or correct the following line: ssl = yes SSL encrypt network traffic between the Dovecot server and its clients protecting user credentials, mail as it is downloaded, and clients may use SSL certificates to authenticate the server, preventing another system from impersonating the server. Headache Chastity
lownsConfigure Dovecot to Use the SSL Certificate fileThis option tells Dovecot where to find the the mail server's SSL Certificate. Edit /etc/dovecot/conf.d/10-ssl.conf and add or correct the following line (note: the path below is the default path set by the Dovecot installation. If you are using a different path, ensure you reference the appropriate file): ssl_cert = </etc/pki/dovecot/certs/dovecot.pem SSL certificates are used by the client to authenticate the identity of the server, as well as to encrypt credentials and message traffic. Not using SSL to encrypt mail server traffic could allow unauthorized access to credentials and mail messages since they are sent in plain text over the network. Canker Sores Diligence
lownsConfigure Dovecot to Use the SSL Key fileThis option tells Dovecot where to find the the mail server's SSL Key. Edit /etc/dovecot/conf.d/10-ssl.conf and add or correct the following line (note: the path below is the default path set by the Dovecot installation. If you are using a different path, ensure you reference the appropriate file): ssl_key = </etc/pki/dovecot/private/dovecot.pem SSL certificates are used by the client to authenticate the identity of the server, as well as to encrypt credentials and message traffic. Not using SSL to encrypt mail server traffic could allow unauthorized access to credentials and mail messages since they are sent in plain text over the network. Obesity Chastity
lowCCE-27144-5Disable Plaintext AuthenticationTo prevent Dovecot from attempting plaintext authentication of clients, edit /etc/dovecot/conf.d/10-auth.conf and add or correct the following line: disable_plaintext_auth = yes Using plain text authentication to the mail server could allow an attacker access to credentials by monitoring network traffic. Baldness Temperance
Allow IMAP Clients to Access the ServerThe default iptables configuration does not allow inbound access to any services. This modification will allow remote hosts to initiate connections to the IMAP daemon, while keeping all other ports on the server in their default protected state. To configure iptables to allow port 143 traffic one must edit /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). Add the following line, ensuring that it appears before the final LOG and DROP lines for the INPUT chain: -A INPUT -m state --state NEW -p tcp --dport 143 -j ACCEPT
Samba(SMB) Microsoft Windows File Sharing ServerWhen properly configured, the Samba service allows Linux machines to provide file and print sharing to Microsoft Windows machines. There are two software packages that provide Samba support. The first, samba-client, provides a series of command line tools that enable a client machine to access Samba shares. The second, simply labeled samba, provides the Samba service. It is this second package that allows a Linux machine to act as an Active Directory server, a domain controller, or as a domain member. Only the samba-client package is installed by default.
Disable Samba if Possible Even after the Samba server package has been installed, it will remain disabled. Do not enable this service unless it is absolutely necessary to provide Microsoft Windows file and print sharing functionality.
lowCCE-27143-7Disable Samba The smb service can be disabled with the following command: # chkconfig smb off Running a Samba server provides a network-based avenue of attack, and should be disabled if not needed. Poison Oak Patience
DISA CCI 1436
Configure Samba if NecessaryAll settings for the Samba daemon can be found in /etc/samba/smb.conf. Settings are divided between a [global] configuration section and a series of user created share definition sections meant to describe file or print shares on the system. By default, Samba will operate in user mode and allow client machines to access local home directories and printers. It is recommended that these settings be changed or that additional limitations be set in place.
lownsDisable Root AccessAdministrators should not use administrator accounts to access Samba file and printer shares. Disable the root user and the wheel administrator group: [share] invalid users = root @wheel If administrator accounts cannot be disabled, ensure that local machine passwords and Samba service passwords do not match. Typically, administrator access is required when Samba must create user and machine accounts and shares. Domain member servers and standalone servers may not need administrator access at all. If that is the case, add the invalid users parameter to [global] instead. Bedwetting Humility
lownsDisable Root AccessBy default, Samba will attempt to negotiate with Microsoft Windows machines to set a common communication protocol. Newer versions of Microsoft Windows may require the use of NTLMv2. NTLMv2 is the preferred protocol for authentication, but since older machines do not support it, Samba has disabled it by default. Enable it with the following: [global] client ntlmv2 auth = yes For the sake of backwards compatibility, most modern Windows machines will still allow other machines to communicate with them over weak protocols such as LANMAN. On Samba, by enabling NTLMv2, you are also disabling LANMAN and NTLMv1. If NTLMv1 is required, it is still possible to individually disable LANMAN. Stress Temperance
Let Domain Controllers Create Machine Trust Accounts On-the-FlyAdd or correct an add machine script entry to the [global] section of /etc/samba/smb.conf to allow Samba to dynamically create Machine Trust Accounts: [global] add machine script = /usr/sbin/useradd -n -g machines -d /dev/null -s /sbin/nologin %u Make sure that the group machines exists. If not, add it with the following command: # /usr/sbin/groupadd machines
Restrict Access to the [IPC$] ShareLimit access to the [IPC$] share so that only machines in your network will be able to connect to it: [IPC$] hosts allow = 192.168.1. 127.0.0.1 hosts deny = 0.0.0.0/0
Restrict File SharingOnly users with local user accounts will be able to log in to Samba shares by default. Shares can be limited to particular users or network addresses. Use the hosts allow and hosts deny directives accordingly, and consider setting the valid users directive to a limited subset of users or to a group of users. Separate each address, user, or user group with a space as follows: [share] hosts allow = 192.168.1. 127.0.0.1 valid users = userone usertwo @usergroup It is also possible to limit read and write access to particular users with the read list and write list options, though the permissions set by the system itself will override these settings. Set the read only attribute for each share to ensure that global settings will not accidentally override the individual share settings. Then, as with the valid users directive, separate each user or group of users with a space: [share] read only = yes write list = userone usertwo @usergroup
lowCCE-26328-5Require Client SMB Packet Signing, if using smbclient To require samba clients running smbclient to use packet signing, add the following to the [global] section of the Samba configuration file: client signing = mandatory Requiring samba clients such as smbclient to use packet signing ensures they can only communicate with servers that support packet signing. Packet signing can prevent man-in-the-middle attacks which modify SMB packets in transit. Hiccups Kindness
lowCCE-26792-2Require Client SMB Packet Signing, if using mount.cifsRequire packet signing of clients who mount Samba shares using the mount.cifs program (e.g., those who specify shares in /etc/fstab). To do so, ensure signing options (either sec=krb5i or sec=ntlmv2i) are used. See the mount.cifs(8) man page for more information. A Samba client should only communicate with servers who can support SMB packet signing. Packet signing can prevent man-in-the-middle attacks which modify SMB packets in transit. Seasonal Affective Disorder Chastity
Restrict Printer SharingBy default, Samba utilizes the CUPS printing service to enable printer sharing with Microsoft Windows workstations. If there are no printers on the local machine, or if printer sharing with Microsoft Windows is not required, disable the printer sharing capability by commenting out the following lines, found in /etc/samba/smb.conf: [global] load printers = yes cups options = raw [printers] comment = All Printers path = /usr/spool/samba browseable = no guest ok = no writable = no printable = yes There may be other options present, but these are the only options enabled and uncommented by default. Removing the [printers] share should be enough for most users. If the Samba printer sharing capability is needed, consider disabling the Samba network browsing capability or restricting access to a particular set of users or network addresses. Set the valid users parameter to a small subset of users or restrict it to a particular group of users with the shorthand @. Separate each user or group of users with a space. For example, under the [printers] share: [printers] valid users = user @printerusers
Proxy ServerA proxy server is a very desirable target for a potential adversary because much (or all) sensitive data for a given infrastructure may flow through it. Therefore, if one is required, the machine acting as a proxy server should be dedicated to that purpose alone and be stored in a physically secure location. The system's default proxy server software is Squid, and provided in an RPM package of the same name.
Disable Squid if PossibleIf Squid was installed and activated, but the system does not need to act as a proxy server, then it should be disabled and removed.
lowCCE-27146-0Disable Squid The squid service can be disabled with the following command: # chkconfig squid off Running proxy server software provides a network-based avenue of attack, and should be removed if not needed. Migraine Kindness
lowCCE-26977-9Uninstall squid Package The squid package can be removed with the following command: # yum erase squid If there is no need to make the proxy server software available, removing it provides a safeguard against its activation. Red Eyes Humility
SNMP ServerThe Simple Network Management Protocol allows administrators to monitor the state of network devices, including computers. Older versions of SNMP were well-known for weak security, such as plaintext transmission of the community string (used for authentication) and usage of easily-guessable choices for the community string.
Disable SNMP Server if PossibleThe system includes an SNMP daemon that allows for its remote monitoring, though it not installed by default. If it was installed and activated, the software should be disabled and removed.
lowCCE-26906-8Disable snmpd Service The snmpd service can be disabled with the following command: # chkconfig snmpd off Running SNMP software provides a network-based avenue of attack, and should be disabled if not needed. Bedwetting Kindness
lowCCE-26332-7Uninstall net-snmp PackageThe net-snmp package provides the snmpd service. The net-snmpd package can be removed with the following command: # yum erase net-snmpd If there is no need to run SNMP server software, removing the package provides a safeguard against its activation. Sunburn Skin Chastity
Configure SNMP Server if NecessaryIf it is necessary to run the snmpd agent on the system, some best practices should be followed to minimize the security risk from the installation. The multiple security models implemented by SNMP cannot be fully covered here so only the following general configuration advice can be offered: use only SNMP version 3 security models and enable the use of authentication and encryptionwrite access to the MIB (Management Information Base) should be allowed only if necessaryall access to the MIB should be restricted following a principle of least privilegenetwork access should be limited to the maximum extent possible including restricting to expected network addresses both in the configuration files and in the system firewall rulesensure SNMP agents send traps only to, and accept SNMP queries only from, authorized management stationsensure that permissions on the snmpd.conf configuration file (by default, in /etc/snmp) are 640 or more restrictiveensure that any MIB files' permissions are also 640 or more restrictive
mediumnsConfigure SNMP Service to Use Only SNMPv3 or Newer Edit /etc/snmp/snmpd.conf, removing any references to v1, v2c, or com2sec. Upon doing that, restart the SNMP service: # service snmpd restart Earlier versions of SNMP are considered insecure, as they potentially allow unauthorized access to detailed system management information. Cramps Temperance
mediumnsEnsure Default Password Is Not Used Edit /etc/snmp/snmpd.conf, remove default community string public. Upon doing that, restart the SNMP service: # service snmpd restart Presence of the default SNMP password enables querying of different system aspects and could result in unauthorized knowledge of the system. Spina Bifida Chastity
Documentation to Support DISA OS SRG MappingThese groups exist to document how the Red Hat Enterprise Linux product meets (or does not meet) requirements listed in the DISA OS SRG, for those cases where Groups or Rules elsewhere in scap-security-guide do not clearly relate.
lownsProduct Meets this Requirement This requirement is a permanent not a finding. No fix is required. Red Hat Enterprise Linux meets this requirement through design and implementation. Red Eyes Temperance
DISA CCI 42, 56, 206, 1084, 66, 85, 86, 185, 223, 171, 172, 1694, 770, 804, 162, 163, 164, 345, 346, 1096, 1111, 1291, 386, 156, 186, 1083, 1082, 1090, 804, 1127, 1128, 1129, 1248, 1265, 1314, 1362, 1368, 1310, 1311, 1328, 1399, 1400, 1427, 1499, 1632, 1693, 1665, 1674
lownsProduct Meets this Requirement This requirement is a permanent not a finding. No fix is required. The Red Hat Enterprise Linux audit system meets this requirement through design and implementation. Burns Humility
DISA CCI 130, 157, 131, 132, 133, 134, 135, 159, 174
lownsProduct Meets this Requirement This requirement is a permanent not a finding. No fix is required. Red Hat Enterprise Linux meets this requirement through design and implementation. Rheumatoid Arthritis Chastity
DISA CCI 34, 35, 99, 154, 226, 802, 872, 1086, 1087, 1089, 1091, 1424, 1426, 1428, 1209, 1214, 1237, 1269, 1338, 1425, 1670
lownsGuidance Does Not Meet this Requirement Due to Impracticality or Scope This requirement is NA. No fix is required. The guidance does not meet this requirement. The requirement is impractical or out of scope. Upset Stomach Kindness
DISA CCI 21, 25, 28, 29, 30, 165, 221, 354, 553, 779, 780, 781, 1009, 1094, 1123, 1124, 1125, 1132, 1135, 1140, 1141, 1142, 1143, 1145, 1147, 1148, 1166, 1295, 1340, 1341, 1350, 1356, 1373, 1374, 1383, 1391, 1392, 1395, 1662
lownsImplementation of the Requirement is Not Supported This requirement is a permanent finding and cannot be fixed. An appropriate mitigation for the system must be implemented but this finding cannot be considered fixed. RHEL6 does not support this requirement. Diabetes Patience
DISA CCI 20, 31, 52, 53, 144, 218, 219, 1158, 1294, 1295, 1500
lownsGuidance Does Not Meet this Requirement Due to Impracticality or Scope This requirement is NA. No fix is required. The guidance does not meet this requirement. The requirement is impractical or out of scope. Headache Diligence
DISA CCI 15, 27, 371, 372, 535, 537, 539, 1682, 370, 37, 24, 1112, 1126, 1143, 1149, 1157, 1159, 1210, 1211, 1274, 1372, 1376, 1377, 1352, 1401, 1555, 1556, 1150
lownsA process for prompt installation of OS updates must exist. Procedures to promptly apply software updates must be established and executed. The Red Hat operating system provides support for automating such a process, by running the yum program through a cron job or by managing the system and its packages through the Red Hat Network or a Satellite Server. This is a manual inquiry about update procedure. Pain Charity
DISA CCI 1232