cis-ubuntu2004-v300-2-4-2-1

DevOps & Security

Ensure access to at is configured

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_20.04_LTS_Benchmark_v3.0.0/cis-ubuntu2004-v300-2-4-2-1/SKILL.md

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Ensure access to at is configured

Profile

Level 1 - Server
Level 1 - Workstation

Description

at is a command-line utility used to schedule a job for later execution

Note: if at is not installed on the system, this section can be skipped

Audit

Verify /etc/at.allow:

Command Line

Exists
Is mode 0640 or more restrictive
Is owned by the user root
Is group owned by the group root - OR - the group daemon

# stat -Lc 'Access: (%a/%A) Owner: (%U) Group: (%G)' /etc/at.allow

Verify the returned value is:

Access: (640/-rw-r-----) Owner: (root) Group: (root)
- OR -
Access: (640/-rw-r-----) Owner: (root) Group: (daemon)

Run the following command to verify either at.deny doesn't exist or is:

Mode 0640 or more restrictive
Owned by the user root
Is group owned by the group root - OR - the group daemon

# [ -e "/etc/at.deny" ] && stat -Lc 'Access: (%a/%A) Owner: (%U) Group: (%G)' /etc/at.deny

Verify either nothing is returned - OR - returned value is one of the following:

Access: (640/-rw-r-----) Owner: (root) Group: (root)
- OR -
Access: (640/-rw-r-----) Owner: (root) Group: (daemon)

Note: On systems where at is configured to use the group daemon, if the group daemon is not set as the owner of at.allow, then at will deny access to all users and you will see an error similar to:

You (<USERNAME>) are not allowed to use this program (at)
See at(1) for more information

Remediation

Run the following script to:

Command Line

Create /etc/at.allow if it doesn't exist
Change owner to user root
Change group owner to group root - OR - group daemon if it exists
Change mode to 640 or more restrictive

#!/usr/bin/env bash
{
  [ ! -e "/etc/at.deny" ] && touch /etc/at.allow
  chmod u=rw,g=wx,o=rwx /etc/at.allow
  if grep -Pq -- '^\h*daemon\:' /etc/group; then
    chown root:daemon /etc/at.allow
  else
    chown root:root /etc/at.allow
  fi
}

IF - /etc/at.deny exists, run the following script to:
Change owner to user root
Change group owner to group root - OR - group daemon if it exists
Change mode to 640 or more restrictive

#!/usr/bin/env bash
{
  if [ -e "/etc/at.deny" ]; then
    chmod u=rw,g=wx,o=rwx /etc/at.deny
    if grep -Pq -- '^\h*daemon\:' /etc/group; then
      chown root:daemon /etc/at.deny
    else
      chown root:root /etc/at.deny
    fi
  fi
}

You (<USERNAME>) are not allowed to use this program (at)
See at(1) for more information

References

NIST SP 800-53 Rev. 5: AC-3. MP-2

CIS Controls

Controls Version	Control	IG 1	IG 2	IG 3
v8	3.3 Configure Data Access Control ListsConfigure data access control lists based on a user's need to know. Apply data access control lists, also known as access permissions, to local and remote file systems, databases, and applications.	●	●	●
v7	14.6 Protect Information through Access Control ListsProtect all information stored on systems with file system, network share, claims, application, or database specific access control lists. These controls will enforce the principle that only authorized individuals should have access to the information based on their need to access the information as a part of their responsibilities.	●	●	●

MITRE ATT&CK Mappings

Techniques / Sub-techniques	Tactics	Mitigations
T1053, T1053.003	TA0002	M1018