Cis Ubuntu1804 V220 5 2 3 8

5.2.3.8 Ensure events that modify user/group information are collected

Profile Applicability

• Level 2 - Server
• Level 2 - Workstation

Description

Record events affecting the modification of user or group information, including that of passwords and old passwords if in use.

• /etc/group - system groups
• /etc/passwd - system users
• /etc/gshadow - encrypted password for each group
• /etc/shadow - system user passwords
• /etc/security/opasswd - storage of old passwords if the relevant PAM module is in use

The parameters in this section will watch the files to see if they have been opened for write or have had attribute changes (e.g. permissions) and tag them with the identifier "identity" in the audit log file.

Rationale

Unexpected changes to these files could be an indication that the system has been compromised and that an unauthorized user is attempting to hide their activities or compromise additional accounts.

Audit

On disk configuration

Run the following command to check the on disk rules:

awk '/^ *-w/ \
&&(/\/etc\/group/ \
 ||/\/etc\/passwd/ \
 ||/\/etc\/gshadow/ \
 ||/\/etc\/shadow/ \
 ||/\/etc\/security\/opasswd/ ) \
&&/ +-p *wa/ \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)/' /etc/audit/rules.d/*.rules

Verify the output matches:

-w /etc/group -p wa -k identity
-w /etc/passwd -p wa -k identity
-w /etc/gshadow -p wa -k identity
-w /etc/shadow -p wa -k identity
-w /etc/security/opasswd -p wa -k identity

Running configuration

Run the following command to check loaded rules:

auditctl -l | awk '/^ *-w/ \
&&(/\/etc\/group/ \
 ||/\/etc\/passwd/ \
 ||/\/etc\/gshadow/ \
 ||/\/etc\/shadow/ \
 ||/\/etc\/security\/opasswd/ ) \
&&(/ key= *[!-~]* *$/||/ -k *[!-~]* *$/)/'

Verify the output matches:

-w /etc/group -p wa -k identity
-w /etc/passwd -p wa -k identity
-w /etc/gshadow -p wa -k identity
-w /etc/shadow -p wa -k identity
-w /etc/security/opasswd -p wa -k identity

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 user/group information.

Example:

printf "
-w /etc/group -p wa -k identity
-w /etc/passwd -p wa -k identity
-w /etc/gshadow -p wa -k identity
-w /etc/shadow -p wa -k identity
-w /etc/security/opasswd -p wa -k identity
" >> /etc/audit/rules.d/50-identity.rules

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.