# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-008
title: Restrict the su command to an empty group
finding:
  title: Any user who knows a target password can switch user with su
  description: su bypasses the sudo rules and logs, and lets anyone who guessed or stole a password become that user.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [su, pam, privilege-escalation]

description: |
  `su` switches to another user, root by default, after asking for that user's password. The
  `pam_wheel` module can restrict it to members of one group. Pointing it at an empty group
  means only root can use `su`; administrators use `sudo` instead.
rationale: |
  Administration through `su` needs a shared root password and leaves no trace of who ran which
  command. Restricting it pushes every privileged action through sudo, with per-user rules and
  logs, and stops password guessing through `su`.
impact: |
  Users can no longer run `su` or `su - otheruser`. Use `sudo -i` or `sudo -u otheruser -i`.
  Root keeps using `su` to switch to any account.

parameters:
  - name: su_group
    type: string
    description: Name of the group allowed to use su. Keep it empty.
    default: sugroup

audit:
  manual: |
    1. `/etc/pam.d/su` must contain an uncommented
       `auth required pam_wheel.so use_uid group={{ su_group }}` line.
    2. Run `getent group {{ su_group }}`. The group must exist and have no members.
  checks:
    - id: pam-wheel
      engine: shell
      description: pam_wheel restricts su to the configured group.
      spec:
        script: |
          grep -Eq "^[[:space:]]*auth[[:space:]]+required[[:space:]]+pam_wheel\.so([[:space:]].*)?[[:space:]]group=$CB_SU_GROUP([[:space:]]|$)" /etc/pam.d/su \
            || { echo "no pam_wheel group=$CB_SU_GROUP line"; exit 1; }
          echo "pam_wheel group=$CB_SU_GROUP"
    - id: group-empty
      engine: shell
      description: The su group exists and is empty.
      spec:
        script: |
          g=$(getent group "$CB_SU_GROUP") || { echo "group $CB_SU_GROUP missing"; exit 1; }
          m=${g##*:}
          echo "members: ${m:-none}"
          [ -z "$m" ]

remediation:
  manual: |
    1. Create the group: `groupadd {{ su_group }}`.
    2. Add to `/etc/pam.d/su`, before the `@include common-auth` line:
       `auth required pam_wheel.so use_uid group={{ su_group }}`
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Remove the line from `/etc/pam.d/su`.
  scripts:
    - id: configure
      engine: shell
      content: |
        getent group "$CB_SU_GROUP" >/dev/null || groupadd "$CB_SU_GROUP"
        f=/etc/pam.d/su
        if ! grep -Eq "^[[:space:]]*auth[[:space:]]+required[[:space:]]+pam_wheel\.so.*group=$CB_SU_GROUP" "$f"; then
          cb_backup "$f"
          sed -i "0,/^@include common-auth/s//auth       required   pam_wheel.so use_uid group=$CB_SU_GROUP\n&/" "$f"
        fi

references:
  - title: pam_wheel(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/pam_wheel.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: su(1), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man1/su.1.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R33)
    url: https://messervices.cyber.gouv.fr/documents-guides/fr_np_linux_configuration-v2.0.pdf
    publisher: ANSSI
    kind: guide
    accessed: 2026-10-19
mappings:
  - framework: anssi-linux
    framework_version: "2.0"
    id: R33
    relation: partial
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-6
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.2.7"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1548
  - framework: mitre-attack
    id: T1110

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: "Fails on a fresh install as expected; generated fix, check again and second fix idempotent; kept after reboot; 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: "Fails on a fresh install as expected; generated fix, check again and second fix idempotent; kept after reboot; 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: "Fails on a fresh install as expected; generated fix, check again and second fix idempotent; kept after reboot; 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: "Fails on a fresh install as expected; generated fix, check again and second fix idempotent; kept after reboot; profiles core and server-hardened."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19