# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-AUTH-019
title: Set a restrictive default umask for login sessions
finding:
  title: New files are readable by every local user by default
  description: With the Ubuntu default umask, documents, exports and logs created by users and root are world-readable.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [accounts, umask, permissions]

description: |
  The umask removes permission bits from every file and directory a process creates. Ubuntu
  login sessions get `022` (files `644`, everyone can read them), and `pam_umask` with
  `USERGROUPS_ENAB yes` even gives `002` to regular users who have their own group. This control sets `{{ umask }}` from
  `/etc/profile.d/`, which applies to the login shells of every user, root included.
rationale: |
  Users and administrators regularly create files with sensitive content (exports, dumps,
  configuration copies) in their home or in `/tmp`. A restrictive umask keeps them private
  unless someone opens them on purpose.
impact: |
  Files created in shared directories are no longer readable by other users. Teams that share
  files through a common group need `chmod g+r` or a setgid group directory.
notes: |
  Services started by systemd do not read `/etc/profile`; set their umask with `UMask=` in their unit.

setting:
  location: /etc/profile.d/60-cirsec-umask.sh
  default_value: "022"
  recommended_value: "{{ umask }}"

parameters:
  - name: umask
    type: enum
    description: Default umask of login shells.
    default: "027"
    allowed: ["027", "077"]
    profile_defaults:
      server-hardened: "077"

audit:
  manual: |
    Run `su -s /bin/sh -l nobody -c umask` and `su -l root -c umask`. Both must print
    `0{{ umask }}` (or a stricter value).
  checks:
    - id: user
      engine: shell
      description: Umask of a login shell for a regular account.
      spec:
        script: |
          u=$(su -s /bin/sh -l nobody -c umask 2>/dev/null | tail -n 1) || exit 3
          echo "nobody login umask: $u"
          [ $(( 0$u & 0$CB_UMASK )) -eq $(( 0$CB_UMASK )) ]
    - id: root
      engine: shell
      description: Umask of a root login shell.
      spec:
        script: |
          u=$(su -l root -c umask 2>/dev/null | tail -n 1) || exit 3
          echo "root login umask: $u"
          [ $(( 0$u & 0$CB_UMASK )) -eq $(( 0$CB_UMASK )) ]

remediation:
  manual: |
    Create `/etc/profile.d/60-cirsec-umask.sh` containing `umask {{ umask }}`. New login sessions
    pick it up.
  effects:
    restart: session
    disruption: low
    lockout_risk: false
  rollback: |
    Delete `/etc/profile.d/60-cirsec-umask.sh`.
  scripts:
    - id: profile
      engine: shell
      content: |
        f=/etc/profile.d/60-cirsec-umask.sh
        cb_backup "$f"
        printf '%s\n' '# Written by CIRSEC Baselines CB-UBU-AUTH-019' "umask $CB_UMASK" >"$f"
        chmod 644 "$f"

references:
  - title: pam_umask(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/pam_umask.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: bash(1), Ubuntu manual pages (umask, INVOCATION)
    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 (R36)
    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: R36
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-3
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.4.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "5.4.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.4.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "5.4.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.4.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "5.4.3.3"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.4.2.6"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "5.4.3.3"
    relation: equivalent
threats:
  - framework: cwe
    id: CWE-276

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 (022); login shells of nobody and root get 027 in core, 077 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 (022); login shells of nobody and root get 027 in core, 077 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 (022); login shells of nobody and root get 027 in core, 077 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 (022); login shells of nobody and root get 027 in core, 077 in server-hardened; kept after reboot."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19