Vulnerability Discussion

Without generating audit records that are specific to the security and mission needs of the organization, it would be difficult to establish, correlate, and investigate the events relating to an incident or identify those responsible for one.

Audit records can be generated from various components within the information system (e.g., module or policy filter). The "chmod" system call changes the file mode bits of each given file according to mode, which can be either a symbolic representation of changes to make or an octal number representing the bit pattern for the new mode bits.
The "fchmod" system call is used to change permissions of a file.
The "fchmodat" system call is used to change permissions of a file relative to a directory file descriptor.

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.

The system call rules are loaded into a matching engine that intercepts each syscall that all programs on the system makes. Therefore, it is very important to only use syscall rules when absolutely necessary since these affect performance. The more rules, the bigger the performance hit. Performance can be helped, though, by combining syscalls into one rule whenever possible.

Satisfies: SRG-OS-000037-GPOS-00015, SRG-OS-000042-GPOS-00020, SRG-OS-000062-GPOS-00031, SRG-OS-000064-GPOS-00033, SRG-OS-000392-GPOS-00172, SRG-OS-000462-GPOS-00206, SRG-OS-000466-GPOS-00210, SRG-OS-000471-GPOS-00215

Check

Verify OL 8 generates an audit record for any use of the "chmod","fchmod", and "fchmodat" syscalls by running the following command to check the file system rules in "/etc/audit/audit.rules":

$ sudo grep chmod /etc/audit/audit.rules

-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_chng
-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_chng

If the command does not return an audit rule for "chmod", "fchmod", and "fchmodat", or any of the lines returned are 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 "chmod", "fchmod", and "fchmodat" syscalls by adding or updating the following line to "/etc/audit/rules.d/audit.rules":

-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_chng
-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -F auid>=1000 -F auid!=unset -k perm_chng

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