# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-012
title: Prevent reuse of recent passwords
finding:
  title: Users can set a password they used before
  description: Users forced to change a password can switch back to the old one, which may already be known to an attacker.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [pam, password]

description: |
  `pam_pwhistory` (in `libpam-modules`) keeps the hashes of each user's last passwords in
  `/etc/security/opasswd` and refuses a new password that matches one of the last `remember`.
  Ubuntu ships the module but does not enable it; a `pam-auth-update` profile adds it to the
  password stack, after the quality check and before `pam_unix`.
rationale: |
  When a password is changed after a suspected compromise, going back to the previous one undoes
  the change. Remembering recent passwords also stops users from cycling between two values.
impact: |
  Users cannot reuse their last {{ remember }} passwords. The history file grows by one hash per
  user and change.
notes: |
  The options are set on the `pam_pwhistory.so` line, which works on every Ubuntu release.
  `/etc/security/pwhistory.conf` only exists from Ubuntu 24.04 (PAM 1.5); options on the module
  line take precedence over it. The history must be kept by `pam_pwhistory` only: the older
  `remember=` option of `pam_unix` stores it in a different way and must not be used.

setting:
  location: /usr/share/pam-configs/cirsec_pwhistory
  default_value: Not enabled
  recommended_value: "remember={{ remember }}, enforce_for_root, use_authtok"

parameters:
  - name: remember
    type: integer
    description: Number of previous passwords that cannot be reused.
    default: 24
    minimum: 5
    maximum: 400

audit:
  manual: |
    `/etc/pam.d/common-password` must contain a `pam_pwhistory.so` line with `remember=` of at
    least {{ remember }}, `enforce_for_root` and `use_authtok`, placed before `pam_unix.so`.
    No `pam_unix.so` line of `/etc/pam.d/common-*` has a `remember=` option.
  checks:
    - id: module
      engine: shell
      description: pam_pwhistory line and its options in common-password.
      spec:
        script: |
          l=$(grep -E '^[[:space:]]*password[[:space:]].*pam_pwhistory\.so' /etc/pam.d/common-password) \
            || { echo "pam_pwhistory not enabled"; exit 1; }
          echo "$l"
          r=$(printf '%s\n' "$l" | sed -n 's/.*remember=\([0-9]*\).*/\1/p')
          [ "${r:-0}" -ge "$CB_REMEMBER" ] && printf '%s\n' "$l" | grep -q enforce_for_root && printf '%s\n' "$l" | grep -q use_authtok
    - id: no-unix-remember
      engine: file-content
      description: pam_unix does not keep its own password history.
      spec: { path: "/etc/pam.d/common-*", pattern: "^[^#]*pam_unix\\.so.*\\sremember=", expect: no_match }

remediation:
  manual: |
    1. Create `/usr/share/pam-configs/cirsec_pwhistory`:
       ```
       Name: CIRSEC - password history (pam_pwhistory)
       Default: yes
       Priority: 1000
       Password-Type: Primary
       Password:
               requisite       pam_pwhistory.so remember={{ remember }} enforce_for_root try_first_pass use_authtok
       ```
    2. Run `pam-auth-update --enable cirsec_pwhistory`.
    3. If a `pam_unix.so` line has `remember=`, remove it from the `pam-auth-update` profile that
       adds it (or from the file edited by hand), then run `pam-auth-update` again.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Run `pam-auth-update --remove cirsec_pwhistory` and delete the profile file.
  scripts:
    - id: configure
      engine: shell
      content: |
        p=/usr/share/pam-configs/cirsec_pwhistory
        printf '%s\n' 'Name: CIRSEC - password history (pam_pwhistory)' 'Default: yes' 'Priority: 1000' \
          'Password-Type: Primary' 'Password:' \
          "	requisite	pam_pwhistory.so remember=$CB_REMEMBER enforce_for_root try_first_pass use_authtok" >"$p"
        cb_backup /etc/pam.d/common-password
        DEBIAN_FRONTEND=noninteractive pam-auth-update --enable cirsec_pwhistory

references:
  - title: pam_pwhistory(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/pam_pwhistory.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: pam-auth-update(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/pam-auth-update.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
mappings:
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: IA-5(1)
    relation: partial
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.3.2.4"
    relation: partial
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.3.3.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.3.3.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.3.3.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.3.3.4.2"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.3.2.4"
    relation: partial
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.3.3.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.3.3.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.3.3.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.3.3.4.2"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.3.2.4"
    relation: partial
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.3.3.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.3.3.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.3.3.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.3.3.4.2"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.3.2.4"
    relation: partial
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.3.3.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.3.3.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.3.3.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.3.3.4.2"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1110
related_controls:
  - id: CB-UBU-AUTH-010
    type: related

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: "Reuse of the previous password rejected with pamtester; no remember on pam_unix; kept after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["22.04"]
    environment: Proxmox VM, Ubuntu 22.04 cloud image, kernel 5.15.0-198-generic
    notes: "Reuse of the previous password rejected with pamtester; no remember on pam_unix; kept after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["24.04"]
    environment: Proxmox VM, Ubuntu 24.04 cloud image, kernel 6.8.0-147-generic
    notes: "Reuse of the previous password rejected with pamtester; no remember on pam_unix; kept after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["26.04"]
    environment: Proxmox VM, Ubuntu 26.04 cloud image, kernel 7.0.0-38-generic
    notes: "Reuse of the previous password rejected with pamtester; no remember on pam_unix; kept after reboot."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19