Vulnerability Discussion

Any script or executable called by cron as root or by any privileged user must be owned by that user, must have the permissions 755 or more restrictive, and should have no extended rights that allow any nonprivileged user to modify the script or executable.

Check

Verify that the Ubuntu operating system is configured to audit the execution of any system call made by cron as root or as any privileged user.

$ sudo auditctl -l | grep /etc/cron.d
-w /etc/cron.d -p wa -k cronjobs

$ sudo auditctl -l | grep /var/spool/cron
-w /var/spool/cron -p wa -k cronjobs

If either of these commands does not return the expected output, or the lines are commented out, this is a finding.

Fix

Configure the Ubuntu operating system to audit the execution of any system call made by cron as root or as any privileged user.

Add or update the following file system rules to "/etc/audit/rules.d/audit.rules":
auditctl -w /etc/cron.d/ -p wa -k cronjobs
auditctl -w /var/spool/cron/ -p wa -k cronjobs

To load the rules to the kernel immediately, use the following command:

$ sudo augenrules --load