# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-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 Debian 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. Debian
  login sessions get `022` (files `644`, everyone can read them); on Debian 13, `pam_umask` with
  `USERGROUPS_ENAB` even gives regular users `002`. 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-DEB-AUTH-019' "umask $CB_UMASK" >"$f"
        chmod 644 "$f"

references:
  - title: pam_umask(8), Debian manual pages
    url: https://manpages.debian.org/trixie/libpam-runtime/pam_umask.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: bash(1), Debian manual pages (umask, INVOCATION)
    url: https://manpages.debian.org/trixie/bash/bash.1.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - 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: 2026-10-18
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-debian-12
    framework_version: "2.0.0"
    id: "5.4.2.6"
    relation: equivalent
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "5.4.3.3"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "5.4.2.6"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "5.4.3.3"
    relation: equivalent
threats:
  - framework: cwe
    id: CWE-276

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