# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-027
title: Keep the user and group databases consistent
finding:
  title: User and group files contain duplicates or dangling references
  description: Duplicate IDs or names give two accounts the same rights and break accountability.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [accounts, integrity]

description: |
  The account databases (`/etc/passwd`, `/etc/shadow`, `/etc/group`, `/etc/gshadow`) must be
  consistent: each user and group name and each UID and GID is unique, every primary group of a
  user exists, and every `passwd` entry has a `shadow` entry. `pwck -r` and `grpck -r` check
  these rules without changing anything.
rationale: |
  Two accounts with the same UID are the same user for the kernel: files, processes and logs
  cannot tell them apart. Duplicates and dangling entries are often the trace of a manual edit
  or of an attacker adding an account.
impact: |
  None. The check is read-only; fixes need a human decision.

audit:
  manual: |
    1. Run `pwck -r` and `grpck -r`. They must report nothing, apart from missing home
       directories of system accounts (`lp`, `news`, `www-data`...), which Ubuntu does not create.
    2. `cut -d: -f3 /etc/passwd | sort | uniq -d` and `cut -d: -f3 /etc/group | sort | uniq -d`
       print nothing.
  checks:
    - id: pwck
      engine: shell
      spec:
        script: |
          out=$(pwck -r 2>&1 | grep -v -e "directory '.*' does not exist" -e '^pwck: no changes$')
          echo "${out:-pwck: no problem (missing system homes ignored)}"
          [ -z "$out" ]
    - id: grpck
      engine: shell
      spec:
        script: |
          out=$(grpck -r 2>&1); rc=$?
          echo "${out:-grpck: no problem}"
          [ "$rc" -eq 0 ]
    - id: duplicate-ids
      engine: shell
      spec:
        script: |
          u=$(cut -d: -f3 /etc/passwd | sort | uniq -d | tr '\n' ' ')
          g=$(cut -d: -f3 /etc/group | sort | uniq -d | tr '\n' ' ')
          echo "duplicate UIDs: ${u:-none}; duplicate GIDs: ${g:-none}"
          [ -z "$u$g" ]

remediation:
  manual: |
    Investigate each reported entry: a duplicate UID 0 or an unknown account may be malicious.
    Fix with `usermod`, `groupmod`, `userdel` and `groupdel`, or interactively with `pwck` and
    `grpck` (without `-r`).

references:
  - title: pwck(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/pwck.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: grpck(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/grpck.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
mappings:
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: IA-2
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "7.2.3"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "7.2.5"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "7.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "7.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "7.2.8"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "7.2.3"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "7.2.5"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "7.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "7.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "7.2.8"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "7.2.3"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "7.2.5"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "7.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "7.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "7.2.8"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "7.2.3"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "7.2.5"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "7.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "7.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "7.2.8"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1136.001

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