Cis Ubuntu1804 V220 5 2 3 3

5.2.3.3 Ensure events that modify the sudo log file are collected

Profile Applicability

• Level 2 - Server
• Level 2 - Workstation

Description

Monitor the sudo log file. If the system has been properly configured to disable the use of the su command and force all administrators to have to log in first and then use sudo to execute privileged commands, then all administrator commands will be logged to /var/log/sudo.log. Any time a command is executed, an audit event will be triggered as the /var/log/sudo.log file will be opened for write and the executed administration command will be written to the log.

Rationale

Changes in /var/log/sudo.log indicate that an administrator has executed a command or the log file itself has been tampered with. Administrators will want to correlate the events written to the audit trail with the records written to /var/log/sudo.log to verify if unauthorized commands have been executed.

Audit

On disk configuration

Run the following command to check the on disk rules:

SUDO_LOG_FILE_ESCAPED=$(grep -r logfile /etc/sudoers* | sed -e 's/.*logfile=//;s/,? .*//' -e 's/"//g' -e 's|/|\\/|g')
[ -n "${SUDO_LOG_FILE_ESCAPED}" ] && awk "/^ *-w/ \
&&/"${SUDO_LOG_FILE_ESCAPED}"/ \
&&/ +-p *wa/ \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)/" /etc/audit/rules.d/*.rules \
|| printf "ERROR: Variable 'SUDO_LOG_FILE_ESCAPED' is unset.\n"

Verify output of matches:

-w /var/log/sudo.log -p wa -k sudo_log_file

Running configuration

Run the following command to check loaded rules:

SUDO_LOG_FILE_ESCAPED=$(grep -r logfile /etc/sudoers* | sed -e 's/.*logfile=//;s/,? .*//' -e 's/"//g' -e 's|/|\\/|g')
[ -n "${SUDO_LOG_FILE_ESCAPED}" ] && auditctl -l | awk "/^ *-w/ \
&&/"${SUDO_LOG_FILE_ESCAPED}"/ \
&&/ +-p *wa/ \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)/" \
|| printf "ERROR: Variable 'SUDO_LOG_FILE_ESCAPED' is unset.\n"

Verify output matches:

-w /var/log/sudo.log -p wa -k sudo_log_file

Remediation

Edit or create a file in the /etc/audit/rules.d/ directory, ending in .rules extension, with the relevant rules to monitor events that modify the sudo log file.

Example:

SUDO_LOG_FILE=$(grep -r logfile /etc/sudoers* | sed -e 's/.*logfile=//;s/,? .*//' -e 's/"//g')
[ -n "${SUDO_LOG_FILE}" ] && printf "
-w ${SUDO_LOG_FILE} -p wa -k sudo_log_file
" >> /etc/audit/rules.d/50-sudo.rules || printf "ERROR: Variable 'SUDO_LOG_FILE_ESCAPED' is unset.\n"

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

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.