# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-028
title: Keep the shadow group empty
finding:
  title: Users are members of the shadow group
  description: Members of the shadow group can read every password hash of the system.
status: active
profiles: [core]
risk:
  severity: high
assessment: automated
tags: [accounts, shadow, password]

description: |
  `/etc/shadow` is readable by the `shadow` group so that a few setgid programs (such as
  `unix_chkpwd`) can check passwords. No user needs to be a member of this group.
rationale: |
  A user in the `shadow` group can copy every password hash and crack them offline.
impact: |
  None for a standard system.

audit:
  manual: |
    1. `getent group shadow` shows no member after the last colon.
    2. `awk -F: -v g="$(getent group shadow | cut -d: -f3)" '$4 == g' /etc/passwd` prints nothing.
  checks:
    - id: members
      engine: shell
      spec:
        script: |
          g=$(getent group shadow) || { echo "no shadow group"; exit 0; }
          gid=$(printf '%s' "$g" | cut -d: -f3)
          m=${g##*:}
          p=$(awk -F: -v g="$gid" '$4 == g {print $1}' /etc/passwd | tr '\n' ' ')
          echo "members: ${m:-none}; primary group of: ${p:-none}"
          [ -z "$m$p" ]

remediation:
  manual: |
    Remove each member with `gpasswd -d <user> shadow`, and give users whose primary group is
    `shadow` another primary group with `usermod -g <group> <user>`.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  scripts:
    - id: remove-members
      engine: shell
      content: |
        cb_backup /etc/group
        cb_backup /etc/gshadow
        m=$(getent group shadow | cut -d: -f4)
        for u in $(printf '%s' "$m" | tr ',' ' '); do gpasswd -d "$u" shadow >/dev/null; done

references:
  - title: shadow(5), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man5/shadow.5.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
mappings:
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-6
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "7.2.4"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "7.2.4"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "7.2.4"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "7.2.4"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1003.008

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["20.04"]
    environment: Proxmox VM, Ubuntu 20.04 cloud image, kernel 5.4.0-216-generic
    notes: "Passes on a fresh install; profiles core and server-hardened."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["22.04"]
    environment: Proxmox VM, Ubuntu 22.04 cloud image, kernel 5.15.0-198-generic
    notes: "Passes on a fresh install; profiles core and server-hardened."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["24.04"]
    environment: Proxmox VM, Ubuntu 24.04 cloud image, kernel 6.8.0-147-generic
    notes: "Passes on a fresh install; profiles core and server-hardened."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["26.04"]
    environment: Proxmox VM, Ubuntu 26.04 cloud image, kernel 7.0.0-38-generic
    notes: "Passes on a fresh install; profiles core and server-hardened."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19