cis-ubuntu1804-v220-5-2-3-6

5.2.3.6 Ensure use of privileged commands are collected

Profile Applicability

• Level 2 - Server
• Level 2 - Workstation

Description

Monitor privileged programs, those that have the setuid and/or setgid bit set on execution, to determine if unprivileged users are running these commands.

Rationale

Execution of privileged commands by non-privileged users could be an indication of someone trying to gain unauthorized access to the system.

Impact

Both the audit and remediation section of this recommendation will traverse all mounted file systems that is not mounted with either noexec or nosuid mount options. If there are large file systems without these mount options, such traversal will be significantly detrimental to the performance of the system.

Before running either the audit or remediation section, inspect the output of the following command to determine exactly which file systems will be traversed:

findmnt -n -l -k -it $(awk '/nodev/ { print $2 }' /proc/filesystems | paste -sd,) | grep -Pv "noexec|nosuid"

To exclude a particular file system due to adverse performance impacts, update the audit and remediation sections by adding a sufficiently unique string to the grep statement. The above command can be used to test the modified exclusions.

Audit

On disk configuration

Run the following command to check on disk rules:

RUNNING=0
PARTITION=$(findmnt -n -l -k -it $(awk '/nodev/ { print $2 }' /proc/filesystems | paste -sd,) | grep -Pv "noexec|nosuid" | awk '{print $1}')
for PARTITION in $(findmnt -n -l -k -it $(awk '/nodev/ { print $2 }' /proc/filesystems | paste -sd,) | grep -Pv "noexec|nosuid" | awk '{print $1}'); do
    for PRIVILEGED in $(find "${PARTITION}" -xdev -perm /6000 -type f); do
        grep -qr "${PRIVILEGED}" /etc/audit/rules.d && printf "OK: '${PRIVILEGED}' found in auditing rules.\n" || printf "Warning: '${PRIVILEGED}' not found in on disk configuration.\n"
    done
done

Verify that all output is OK.

Running configuration

Run the following command to check loaded rules:

RUNNING=$(auditctl -l)
[ -n "${RUNNING}" ] && for PARTITION in $(findmnt -n -l -k -it $(awk '/nodev/ { print $2 }' /proc/filesystems | paste -sd,) | grep -Pv "noexec|nosuid" | awk '{print $1}'); do
    for PRIVILEGED in $(find "${PARTITION}" -xdev -perm /6000 -type f); do
        printf -- "${RUNNING}" | grep -q "${PRIVILEGED}" && printf "OK: '${PRIVILEGED}' found in auditing rules.\n" || printf "Warning: '${PRIVILEGED}' not found in running configuration.\n"
    done
done \
|| printf "ERROR: Variable 'RUNNING' is unset.\n"

Verify that all output is OK.

Special mount points

If there are any special mount points that are not visible by default from findmnt as per the above audit, said file systems would have to be manually audited.

Remediation

Edit or create a file in the /etc/audit/rules.d/ directory, ending in .rules extension, with the relevant rules to monitor the use of privileged commands.

Example:

UID_MIN=$(awk '/^\s*UID_MIN/{print $2}' /etc/login.defs)
AUDIT_RULE_FILE="/etc/audit/rules.d/50-privileged.rules"
NEW_DATA=()
for PARTITION in $(findmnt -n -l -k -it $(awk '/nodev/ { print $2 }' /proc/filesystems | paste -sd,) | grep -Pv "noexec|nosuid" | awk '{print $1}'); do
    readarray -t DATA < <(find "${PARTITION}" -xdev -perm /6000 -type f | awk -v UID_MIN=${UID_MIN} '{print "-a always,exit -F path=" $1 " -F perm=x -F auid>="UID_MIN" -F auid!=unset -k privileged" }')
    for ENTRY in "${DATA[@]}"; do
        NEW_DATA+=("${ENTRY}")
    done
done
readarray &5 /dev/null -t OLD_DATA < "${AUDIT_RULE_FILE}"
COMBINED_DATA=( "${OLD_DATA[@]}" "${NEW_DATA[@]}" )
printf '%s\n' "${COMBINED_DATA[@]}" | sort -u > "${AUDIT_RULE_FILE}"

Merge and load the rules into active configuration:

augenrules --load

Check if reboot is required:

if [[ $(auditctl -s | grep "enabled") =~ "2" ]]; then printf "Reboot required to load rules\n"; fi

Special mount points

If there are any special mount points that are not visible by default from just scanning /, change the PARTITION variable to the appropriate partition and re-run the remediation.

References

• NIST SP 800-53 Rev. 5: AU-3

CIS Controls

MITRE ATT&CK Mappings

Additional Information

Potential reboot required: If the auditing configuration is locked (-e 2), then augenrules will not warn in any way that rules could not be loaded into the running configuration. A system reboot will be required to load the rules into the running configuration.

System call structure: For performance (man 7 audit.rules) reasons it is preferable to have all the system calls on one line. However, your configuration may have them on one line each or some other combination.