# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-PKG-004
title: Restrict write access to APT keys and repository lists
finding:
  title: Non-root users can change the APT keys or repository lists
  description: Whoever can write these files chooses which repositories and signing keys APT trusts, and so which code runs as root.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [apt, repositories, permissions, supply-chain]

description: |
  APT trusts the keyrings in `/etc/apt/trusted.gpg.d/`, `/etc/apt/keyrings/`,
  `/usr/share/keyrings/` (and the legacy `/etc/apt/trusted.gpg`), and downloads from the
  repositories listed in `/etc/apt/sources.list` and `/etc/apt/sources.list.d/`. These
  directories should be `root:root` mode `0755` and their files `root:root` mode `0644` or
  stricter.
rationale: |
  A user able to add a key or a source entry can make APT install a package of their choice, with
  maintainer scripts running as root, at the next update. Only root may change them. Reading them
  is harmless: keys are public and APT's unprivileged `_apt` user needs to read them.
impact: |
  None known. Ubuntu ships these files with these permissions.
notes: |
  `/usr/share/keyrings/` also holds the Ubuntu Pro keys (`ubuntu-pro-*.gpg`) used by the ESM
  repositories. `/etc/apt/keyrings/` does not exist on 20.04 (not applicable until created). The
  check follows symbolic links and checks their target.

setting:
  location: /etc/apt/trusted.gpg.d, /etc/apt/keyrings, /usr/share/keyrings, /etc/apt/sources.list, /etc/apt/sources.list.d
  default_value: "root:root, 0755 for directories and 0644 for files"
  recommended_value: "root:root, 0755 or stricter for directories, 0644 or stricter for files"

audit:
  manual: |
    1. `stat -c '%U:%G %a %n' /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings /etc/apt/sources.list.d`
       shows `root:root` and `755` or stricter.
    2. `stat -L -c '%U:%G %a %n' /etc/apt/trusted.gpg /etc/apt/trusted.gpg.d/* /etc/apt/keyrings/* /usr/share/keyrings/* /etc/apt/sources.list /etc/apt/sources.list.d/*`
       shows `root:root` and `644` or stricter for every existing file.
  checks:
    - id: trusted-gpg-d
      engine: file-perms
      description: The /etc/apt/trusted.gpg.d directory.
      spec: { path: /etc/apt/trusted.gpg.d, owner: root, group: root, mode: "0755" }
    - id: etc-keyrings
      engine: file-perms
      description: The /etc/apt/keyrings directory.
      spec: { path: /etc/apt/keyrings, owner: root, group: root, mode: "0755" }
    - id: usr-share-keyrings
      engine: file-perms
      description: The /usr/share/keyrings directory.
      spec: { path: /usr/share/keyrings, owner: root, group: root, mode: "0755" }
    - id: key-files
      engine: shell
      description: Every keyring file APT can use, following symbolic links.
      spec:
        script: |
          d="/etc/apt/trusted.gpg /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings"
          # shellcheck disable=SC2086
          bad=$(find -L $d -maxdepth 1 -type f \( ! -user root -o ! -group root -o -perm /7133 \) \
                -exec stat -L -c '%n %U:%G %a' {} + 2>/dev/null)
          if [ -n "$bad" ]; then
            printf '%s\n' "$bad"
            exit 1
          fi
          # shellcheck disable=SC2086
          echo "$(find -L $d -maxdepth 1 -type f 2>/dev/null | wc -l) key files root:root 644 or stricter"
    - id: sources-list
      engine: file-perms
      description: The /etc/apt/sources.list file.
      spec: { path: /etc/apt/sources.list, owner: root, group: root, mode: "0644" }
    - id: sources-list-d
      engine: file-perms
      description: The /etc/apt/sources.list.d directory.
      spec: { path: /etc/apt/sources.list.d, owner: root, group: root, mode: "0755" }
    - id: sources-list-d-files
      engine: file-perms
      description: Files in /etc/apt/sources.list.d.
      spec: { path: "/etc/apt/sources.list.d/*", owner: root, group: root, mode: "0644" }

remediation:
  manual: |
    1. `chown root:root /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings /etc/apt/sources.list.d`
       and `chmod go-w` on them.
    2. For each key file and source file: `chown root:root <file>` and `chmod 644 <file>` (or stricter).
  effects:
    restart: none
    disruption: none
    lockout_risk: false
  rollback: |
    Restore the previous owner and mode with `chown` and `chmod`. Nothing else depends on them.
  scripts:
    - id: trusted-gpg-d
      from_check: trusted-gpg-d
    - id: etc-keyrings
      from_check: etc-keyrings
    - id: usr-share-keyrings
      from_check: usr-share-keyrings
    - id: key-files
      engine: shell
      content: |
        find -L /etc/apt/trusted.gpg /etc/apt/trusted.gpg.d /etc/apt/keyrings /usr/share/keyrings \
          -maxdepth 1 -type f \( ! -user root -o ! -group root -o -perm /7133 \) 2>/dev/null |
          while IFS= read -r f; do
            chown root:root "$f" && chmod u-xs,go-wxs,o-t "$f" || exit 1
          done
    - id: sources-list
      from_check: sources-list
    - id: sources-list-d
      from_check: sources-list-d
    - id: sources-list-d-files
      from_check: sources-list-d-files

references:
  - title: apt-secure(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/apt-secure.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: sources.list(5), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man5/sources.list.5.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: Ubuntu Server documentation - Third-party repository usage
    url: https://documentation.ubuntu.com/server/explanation/software/third-party-repository-usage/
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
mappings:
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-5
    relation: related
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-3
    relation: related
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.2.1.3"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.2.1.4"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.2.1.7"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.2.1.8"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.2.1.9"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.2.1.3"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.2.1.4"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.2.1.7"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.2.1.8"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.2.1.9"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1195.002
  - framework: cwe
    id: CWE-732
related_controls:
  - id: CB-UBU-PKG-005
    type: related

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 stock; world-writable keyring and 0777 sources.list.d detected and fixed."
  - 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 stock; world-writable keyring and 0777 sources.list.d detected and fixed."
  - 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 stock; world-writable keyring and 0777 sources.list.d detected and fixed."
  - 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 stock; world-writable keyring and 0777 sources.list.d detected and fixed."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19