# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-020
title: Close idle shell sessions automatically
finding:
  title: Idle shell sessions stay open forever
  description: A forgotten terminal, often with a root shell, can be used by anyone who reaches it.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [accounts, session, timeout]

description: |
  When the read-only `TMOUT` variable is set, bash and dash close an interactive shell that
  waited that many seconds for input. Setting it in `/etc/profile.d/` applies it to the login
  shells of every user, and `readonly` stops users from removing it.
rationale: |
  Open terminals left on a console, in a jump host session or in a forgotten SSH connection are
  an easy way to reuse someone's access, sometimes with root privileges.
impact: |
  Idle interactive shells exit after {{ tmout }} seconds. Long-running commands are not
  affected, but a shell waiting at the prompt is closed: use `tmux` or `screen` for sessions
  that must stay open.

setting:
  location: /etc/profile.d/60-cirsec-tmout.sh
  default_value: Not set
  recommended_value: "TMOUT={{ tmout }}, readonly"

parameters:
  - name: tmout
    type: integer
    description: Idle time before an interactive shell exits.
    unit: seconds
    default: 900
    profile_defaults:
      server-hardened: 600
      desktop-hardened: 600
    minimum: 60
    maximum: 3600

audit:
  manual: |
    Run `su -s /bin/sh -l nobody -c 'echo $TMOUT'`. It must print a value between 1 and {{ tmout }}.
  checks:
    - id: login-shell
      engine: shell
      description: TMOUT in a login shell.
      spec:
        script: |
          t=$(su -s /bin/sh -l nobody -c 'printf %s "${TMOUT:-}"' 2>/dev/null) || exit 3
          echo "TMOUT=${t:-unset}"
          [ -n "$t" ] && [ "$t" -ge 1 ] && [ "$t" -le "$CB_TMOUT" ]

remediation:
  manual: |
    Create `/etc/profile.d/60-cirsec-tmout.sh`:
    ```
    TMOUT={{ tmout }}
    readonly TMOUT
    export TMOUT
    ```
  effects:
    restart: session
    disruption: low
    lockout_risk: false
  rollback: |
    Delete `/etc/profile.d/60-cirsec-tmout.sh`.
  scripts:
    - id: profile
      engine: shell
      content: |
        f=/etc/profile.d/60-cirsec-tmout.sh
        cb_backup "$f"
        printf '%s\n' '# Written by CIRSEC Baselines CB-UBU-AUTH-020' "TMOUT=$CB_TMOUT" 'readonly TMOUT' 'export TMOUT' >"$f"
        chmod 644 "$f"

references:
  - title: bash(1), Ubuntu manual pages (TMOUT)
    url: https://manpages.ubuntu.com/manpages/noble/man1/bash.1.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R32)
    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: R32
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-12
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.4.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.4.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.4.3.2"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.4.3.2"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1078

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: "Fails on a fresh install; TMOUT 900 in core, 600 in server-hardened; 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: "Fails on a fresh install; TMOUT 900 in core, 600 in server-hardened; 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: "Fails on a fresh install; TMOUT 900 in core, 600 in server-hardened; 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: "Fails on a fresh install; TMOUT 900 in core, 600 in server-hardened; kept after reboot."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19