Allow only listed users to schedule jobs with crontab and at

Description

crontab and at read an allow list and a deny list:

• /etc/cron.allow and /etc/cron.deny for crontab;
• /etc/at.allow and /etc/at.deny for at.

When the allow file exists, only the users it lists may schedule jobs, and the deny file is ignored. The allow file must exist, belong to root, with group root or the group the tool runs as (crontab for cron, daemon for at), and mode 0640 or stricter. A deny file, if present, must have the same ownership and mode.

The checks run only for the tools that are installed (facts cron and at). Ubuntu installs cron everywhere, and at only on 20.04 (pulled in by the ubuntu-server metapackage). crontab and at read these files through their setgid group, so a group other than root must be crontab or daemon for them to work.

Rationale

An allow list is safer than a deny list: a new account cannot schedule jobs until someone decides it should. Scheduled jobs are a common persistence technique, and restricting who can create them reduces that option for a compromised account.

Impact

Users who are not listed in the allow file can no longer create or change crontabs or at jobs. Add the accounts that need them (service accounts included) before applying. The fix creates missing allow files with root as the only entry.

Audit

1. Run stat -Lc '%n %U:%G %a' /etc/cron.allow /etc/at.allow. Each allow file must exist, belong to root, with group root or crontab (cron) / daemon (at), and mode 640 or stricter.
2. /etc/cron.deny and /etc/at.deny, if they exist, must meet the same rules.

Automated: every check must pass. Scanners report each check by its id.

cron-allow

/etc/cron.allow exists with safe ownership and mode.

script: |
  f=/etc/cron.allow
  [ -f "$f" ] || { echo "$f missing"; exit 1; }
  set -- $(stat -Lc '%U %G %a' "$f")
  echo "$f $1:$2 $3"
  [ "$1" = root ] && { [ "$2" = root ] || [ "$2" = crontab ]; } && [ $((0$3 & 0137)) -eq 0 ]

cron-deny

/etc/cron.deny is absent or has safe ownership and mode.

script: |
  f=/etc/cron.deny
  [ -e "$f" ] || { echo "$f absent"; exit 0; }
  set -- $(stat -Lc '%U %G %a' "$f")
  echo "$f $1:$2 $3"
  [ "$1" = root ] && { [ "$2" = root ] || [ "$2" = crontab ]; } && [ $((0$3 & 0137)) -eq 0 ]

at-allow

/etc/at.allow exists with safe ownership and mode.

script: |
  f=/etc/at.allow
  [ -f "$f" ] || { echo "$f missing"; exit 1; }
  set -- $(stat -Lc '%U %G %a' "$f")
  echo "$f $1:$2 $3"
  [ "$1" = root ] && { [ "$2" = root ] || [ "$2" = daemon ]; } && [ $((0$3 & 0137)) -eq 0 ]

at-deny

/etc/at.deny is absent or has safe ownership and mode.

script: |
  f=/etc/at.deny
  [ -e "$f" ] || { echo "$f absent"; exit 0; }
  set -- $(stat -Lc '%U %G %a' "$f")
  echo "$f $1:$2 $3"
  [ "$1" = root ] && { [ "$2" = root ] || [ "$2" = daemon ]; } && [ $((0$3 & 0137)) -eq 0 ]

Remediation

1. cron: printf 'root\n' > /etc/cron.allow, add the other allowed users one per line, then chown root:crontab /etc/cron.allow and chmod 640 /etc/cron.allow. Apply the same owner and mode to /etc/cron.deny if it exists.
2. at: same steps for /etc/at.allow and /etc/at.deny with group daemon.

Rollback

Delete /etc/cron.allow and /etc/at.allow (the deny files then apply again).

cron

f=/etc/cron.allow
if [ ! -e "$f" ]; then
  cb_backup "$f"
  printf 'root\n' >"$f"
  chown root:crontab "$f"
  chmod 0640 "$f"
fi
for f in /etc/cron.allow /etc/cron.deny; do
  [ -e "$f" ] || continue
  set -- $(stat -Lc '%U %G %a' "$f")
  [ "$1" = root ] || chown root "$f"
  [ "$2" = root ] || [ "$2" = crontab ] || chgrp crontab "$f"
  [ $((0$3 & 0137)) -eq 0 ] || chmod u-x,g-wx,o-rwx "$f"
done

at

f=/etc/at.allow
if [ ! -e "$f" ]; then
  cb_backup "$f"
  printf 'root\n' >"$f"
  chown root:daemon "$f"
  chmod 0640 "$f"
fi
for f in /etc/at.allow /etc/at.deny; do
  [ -e "$f" ] || continue
  set -- $(stat -Lc '%U %G %a' "$f")
  [ "$1" = root ] || chown root "$f"
  [ "$2" = root ] || [ "$2" = daemon ] || chgrp daemon "$f"
  [ $((0$3 & 0137)) -eq 0 ] || chmod u-x,g-wx,o-rwx "$f"
done

Handwritten code reads parameters from CB_<NAME> environment variables. Each release turns these fixes into remediate.sh and an Ansible role per profile: dry run by default, a fix runs only when its check fails.

References

• crontab(1), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19
• at(1), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19

Mappings

How this control relates to the other framework's: equivalent, stricter, looser, partial or related.

Threats mitigated

People and lifecycle