# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-003
title: Protect the login banner files from modification
finding:
  title: Login banner files can be changed by non-root users
  description: Someone who can rewrite the banners can remove the legal notice or plant misleading instructions for users.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [banner, login, permissions]

description: |
  `/etc/issue`, `/etc/issue.net` and `/etc/motd` are shown to users before or after login. They
  must be owned by root and writable only by root.
rationale: |
  A writable banner lets an attacker remove the legal warning, or display fake instructions that
  trick users into running commands or giving out credentials.
impact: |
  None known. These are the Ubuntu defaults when the files exist; Ubuntu ships no `/etc/motd`.

setting:
  location: /etc/issue, /etc/issue.net, /etc/motd
  default_value: "root:root 0644"
  recommended_value: "root:root, mode 0644 or stricter"

audit:
  manual: |
    Run `stat -c '%n %U:%G %a' /etc/issue /etc/issue.net /etc/motd`.
    Each file must be `root:root` with mode `644` or stricter.
  checks:
    - id: issue
      engine: file-perms
      spec: { path: /etc/issue, owner: root, group: root, mode: "0644" }
    - id: issue-net
      engine: file-perms
      spec: { path: /etc/issue.net, owner: root, group: root, mode: "0644" }
    - id: motd
      engine: file-perms
      spec: { path: /etc/motd, owner: root, group: root, mode: "0644" }

remediation:
  manual: |
    Run `chown root:root /etc/issue /etc/issue.net /etc/motd` and
    `chmod 644 /etc/issue /etc/issue.net /etc/motd`.
  effects:
    restart: none
    disruption: none
    lockout_risk: false
  scripts:
    - id: issue
      from_check: issue
    - id: issue-net
      from_check: issue-net
    - id: motd
      from_check: motd

references:
  - title: issue(5), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man5/issue.5.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R50)
    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: R50
    relation: partial
    note: R50 covers every sensitive file; this control covers the login banners.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-3
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "1.6.4"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "1.6.5"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "1.6.6"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "1.6.4"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "1.6.5"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "1.6.6"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.6.6"
    relation: partial
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.6.7"
    relation: partial
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.6.8"
    relation: partial
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.6.9"
    relation: partial
    note: Ours checks /etc/motd, not the files of the dynamic message of the day.
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.6.10"
    relation: partial
    note: Ours checks /etc/issue.net, the default SSH banner of CB-UBU-NET-020.
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.6.6"
    relation: partial
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.6.7"
    relation: partial
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.6.8"
    relation: partial
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.6.9"
    relation: partial
    note: Ours checks /etc/motd, not the files of the dynamic message of the day.
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.6.10"
    relation: partial
    note: Ours checks /etc/issue.net, the default SSH banner of CB-UBU-NET-020.
threats:
  - framework: cwe
    id: CWE-732

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 (/etc/motd absent, not applicable); 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 (/etc/motd absent, not applicable); 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 (/etc/motd absent, not applicable); 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 (/etc/motd absent, not applicable); profiles core and server-hardened."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19