Description

The shadow group allows system programs which require access the ability to read the /etc/shadow file. No users should be assigned to the shadow group.

Rationale

Any users assigned to the shadow group would be granted read access to the /etc/shadow file. If attackers can gain read access to the /etc/shadow file, they can easily run a password cracking program against the hashed passwords to break them. Other security information that is stored in the /etc/shadow file (such as expiration) could also be useful to subvert additional user accounts.

Remediation

Shell script

The following script can be run on the host to remediate the issue.

#!/bin/bash

sed -ri 's/(^shadow:[^:]*:[^:]*:)([^:]+$)/\1/' /etc/group

Ansible playbook

The following playbook can be run with Ansible to remediate the issue.

- name: Ensure interactive local users are the owners of their respective initialization
    files
  ansible.builtin.lineinfile:
    dest: /etc/group
    backrefs: true
    regexp: (^shadow:[^:]*:[^:]*:)([^:]+$)
    line: \1
  tags:
  - PCI-DSS-Req-8.2.1
  - PCI-DSSv4-8.3
  - PCI-DSSv4-8.3.2
  - ensure_shadow_group_empty
  - low_complexity
  - medium_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy

Warning

This rule remediation will ensure the group membership is empty in /etc/group. To avoid any disruption the remediation won’t change the primary group of users in /etc/passwd if any user has the shadow GID as primary group.