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CIS Ubuntu 14.04 LTS - 5.1.8 Ensure at/cron is restricted to authorized users

DevOps & Security
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Verify at and cron access is restricted using allow files with proper ownership and permissions

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5.1.8 Ensure at/cron is restricted to authorized users (Scored)

Profile Applicability

  • Level 1 - Server
  • Level 1 - Workstation

Description

Configure /etc/cron.allow and /etc/at.allow to allow specific users to use these services. If /etc/cron.allow or /etc/at.allow do not exist, then /etc/at.deny and /etc/cron.deny are checked. Any user not specifically defined in those files is allowed to use at and cron. By removing the files, only users in /etc/cron.allow and /etc/at.allow are allowed to use at and cron. Note that even though a given user is not listed in cron.allow, cron jobs can still be run as that user. The cron.allow file only controls administrative access to the crontab command for scheduling and modifying cron jobs.

Rationale

On many systems, only the system administrator is authorized to schedule cron jobs. Using the cron.allow file to control who can run cron jobs enforces this policy. It is easier to manage an allow list than a deny list. In a deny list, you could potentially add a user ID to the system and forget to add it to the deny files.

Audit Procedure

Run the following commands and ensure /etc/cron.deny and /etc/at.deny do not exist:

stat /etc/cron.deny
stat /etc/at.deny

Run the following command and verify Uid and Gid are both 0/root and Access does not grant permissions to group or other for both /etc/cron.allow and /etc/at.allow:

stat /etc/cron.allow
stat /etc/at.allow

Expected Result

stat: cannot stat '/etc/cron.deny': No such file or directory
stat: cannot stat '/etc/at.deny': No such file or directory

Access: (0600/-rw-------)  Uid: (    0/    root)   Gid: (    0/    root)  # for cron.allow
Access: (0600/-rw-------)  Uid: (    0/    root)   Gid: (    0/    root)  # for at.allow

Remediation

Run the following commands to remove /etc/cron.deny and /etc/at.deny and create and set permissions and ownership for /etc/cron.allow and /etc/at.allow:

rm /etc/cron.deny
rm /etc/at.deny
touch /etc/cron.allow
touch /etc/at.allow
chmod og-rwx /etc/cron.allow
chmod og-rwx /etc/at.allow
chown root:root /etc/cron.allow
chown root:root /etc/at.allow

Default Value

Not configured by default.

References

  • CIS Controls: 16 - Account Monitoring and Control

Profile

  • Level 1 - Server
  • Level 1 - Workstation