# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-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), Debian manual pages
    url: https://manpages.debian.org/trixie/libpam-runtime/pam_wheel.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: su(1), Debian manual pages
    url: https://manpages.debian.org/trixie/util-linux/su.1.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - 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: 2026-10-18
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-debian-12
    framework_version: "2.0.0"
    id: "5.2.7"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "5.2.7"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1548
  - framework: mitre-attack
    id: T1110

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["12"]
    environment: Proxmox VM, Debian 12 genericcloud image, kernel 6.1.0-54-cloud-amd64
    notes: Generated check, fix and check again for profiles core and server-hardened; state kept after reboot.
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["13"]
    environment: Proxmox VM, Debian 13 genericcloud image, kernel 6.12.113+deb13-cloud-amd64
    notes: Generated check, fix and check again for profiles core and server-hardened; state kept after reboot.
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18