There are ongoing log entries from cron about the root account that look like:
Cron: pam_unix (cron:session): session opened/closed for user root by (uid=0)
To remove those entries from the log I found the following solution on the web:
session [success=1 default=ignore] pam_succeed_if.so service in cron quiet use_uid
session required pam_unix.so
Right before the line session required pam_unix.so you need to add the line:
session [success=1 default=ignore] pam_succeed_if.so service in cron quiet use_uid