Vulnerability Discussion

Reconstruction of harmful events or forensic analysis is not possible if audit records do not contain enough information.

At a minimum, the organization must audit the full-text recording of privileged commands. The organization must maintain audit trails in sufficient detail to reconstruct events to determine the cause and impact of compromise. The "postdrop" command creates a file in the maildrop directory and copies its standard input to the file.

When a user logs on, the AUID is set to the UID of the account that is being authenticated. Daemons are not user sessions and have the loginuid set to "-1". The AUID representation is an unsigned 32-bit integer, which equals "4294967295". The audit system interprets "-1", "4294967295", and "unset" in the same way.

Satisfies: SRG-OS-000037-GPOS-00015, SRG-OS-000042-GPOS-00020, SRG-OS-000062-GPOS-00031, SRG-OS-000392-GPOS-00172, SRG-OS-000462-GPOS-00206, SRG-OS-000471-GPOS-00215

Check

Verify OL 8 generates an audit event for any use of the "postdrop" command by running the following command to check the file system rules in "/etc/audit/audit.rules":

$ sudo grep -w "postdrop" /etc/audit/audit.rules

-a always,exit -F path=/usr/sbin/postdrop -F perm=x -F auid>=1000 -F auid!=unset -k privileged-unix-update

If the command does not return a line or the line is commented out, this is a finding.

Note: The "-k" allows for specifying an arbitrary identifier, and the string after it does not need to match the example output above.

Fix

Configure the audit system to generate an audit event for any use of the "postdrop" command by adding or updating the following rule in the "/etc/audit/rules.d/audit.rules" file:

-a always,exit -F path=/usr/sbin/postdrop -F perm=x -F auid>=1000 -F auid!=unset -k privileged-unix-update

The audit daemon must be restarted for the changes to take effect. To restart the audit daemon, run the following command:

$ sudo service auditd restart