We can restrict access to the at and cron utilities using /etc/at.allow, /etc/at.deny, /etc/cron.allow and /etc/cron.deny. If the files don't exist then we can create them; the /etc/at.allow and /etc/cron.allow files are sufficient. For the /etc/at.allow file, we do the following:
root@philip-virtual-machine:/home/philip# cat /etc/at.allow
cat: /etc/at.alow: No such file or directory
root@philip-virtual-machine:/home/philip# cat /etc/cron.allow
cat: /etc/cron.allow: No such file or directory
We can use an editor and create the file and store the usernames, one username per line:
root@philip-virtual-machine:/home/philip# cat /etc/at.allow
philip
harry
teddy
root@philip-virtual-machine:/home/philip#
root@philip-virtual-machine:/home/philip# cat /etc/cron.allow
philip
harry
teddy
root@philip-virtual-machine:/home/philip#
Awesome! Now, only these...