# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-021
title: Do not list nologin as a valid shell
finding:
  title: nologin is declared as a valid login shell
  description: Programs that only allow accounts with a valid shell (FTP servers, chsh) treat no-login accounts as normal ones.
status: active
profiles: [server-hardened, desktop-hardened]
risk:
  severity: low
assessment: automated
tags: [accounts, shells]

description: |
  `/etc/shells` lists the valid login shells. Some programs, like `chsh` and FTP servers, use it
  to decide whether an account may log in. `nologin` exists precisely to refuse logins and must
  not appear there.
rationale: |
  Listing `nologin` makes service accounts look like regular users to programs that rely on
  `/etc/shells`, which can open them to FTP logins or let users switch to it and back.
impact: |
  None known. Ubuntu does not list it by default.

audit:
  manual: |
    Run `grep nologin /etc/shells`. There must be no match.
  checks:
    - id: shells
      engine: file-content
      spec: { path: /etc/shells, pattern: "nologin", expect: no_match }

remediation:
  manual: |
    Remove the `nologin` lines from `/etc/shells`.
  effects:
    restart: none
    disruption: none
    lockout_risk: false
  scripts:
    - id: remove
      engine: shell
      content: |
        cb_backup /etc/shells
        sed -i '/nologin/d' /etc/shells

references:
  - title: shells(5), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man5/shells.5.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
mappings:
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-6
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.4.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.4.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.4.3.1"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.4.3.1"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1078.003

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; profile 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; profile 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; profile 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; profile server-hardened."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19