# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-AUTH-002
title: Keep system details out of the message of the day
finding:
  title: The message of the day shows the kernel version to every user who logs in
  description: Every login session, including a compromised account, learns the exact kernel release.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [banner, login, motd]

description: |
  After login, `pam_motd` shows `/etc/motd` and the output of the executable scripts in
  `/etc/update-motd.d/`. Debian ships `10-uname`, which prints the kernel name, release,
  version and architecture.
rationale: |
  The kernel release tells an attacker who reached a shell which local privilege escalation
  exploits apply. There is no need to show it to every user at every login.
impact: |
  Users no longer see the kernel version at login. They can still run `uname -a`.

setting:
  location: /etc/motd, /etc/update-motd.d/10-uname
  default_value: 10-uname executable
  recommended_value: 10-uname not executable, /etc/motd without OS details

audit:
  manual: |
    1. `/etc/motd` must not contain the escape sequences `\m`, `\r`, `\s`, `\v` or the OS name.
    2. `/etc/update-motd.d/10-uname` must not be executable (`ls -l`).
  checks:
    - id: motd-no-os
      engine: file-content
      description: /etc/motd does not reveal the system version.
      spec: { path: /etc/motd, pattern: "\\\\[mrsv]|Debian GNU/Linux [0-9]", expect: no_match }
    - id: uname-script
      engine: file-perms
      description: The 10-uname dynamic MOTD script is not executable.
      spec: { path: /etc/update-motd.d/10-uname, owner: root, group: root, mode: "0644" }

remediation:
  manual: |
    1. Run `chmod 644 /etc/update-motd.d/10-uname`.
    2. Remove OS names, versions and escape sequences from `/etc/motd`.
  effects:
    restart: none
    disruption: none
    lockout_risk: false
  rollback: |
    Run `chmod 755 /etc/update-motd.d/10-uname`.
  scripts:
    - id: uname-script
      from_check: uname-script

references:
  - title: pam_motd(8), Debian manual pages
    url: https://manpages.debian.org/trixie/libpam-runtime/pam_motd.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: motd(5), Debian manual pages
    url: https://manpages.debian.org/trixie/manpages/motd.5.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
mappings:
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-8
    relation: related
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.6.1"
    relation: stricter
    note: Ours also disables the dynamic uname script.
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.6.4"
    relation: partial
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.6.1"
    relation: stricter
    note: Ours also disables the dynamic uname script.
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.6.4"
    relation: partial
threats:
  - framework: mitre-attack
    id: T1082

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