# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-BOOT-007
title: Protect hard links to files the user does not own
finding:
  title: Users can create hard links to files they do not own
  description: Hard links to privileged files enable link-based races and keep vulnerable binaries reachable after an update.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [kernel, sysctl, filesystem, toctou]

description: |
  With `fs.protected_hardlinks = 1`, a user can only create a hard link to a file they own, or
  to a file they can both read and write. Linking to other files needs the `CAP_FOWNER` capability.
rationale: |
  Hard links to files owned by other users enable races similar to symlink attacks. They also
  let an attacker keep a copy of a vulnerable setuid binary that survives its replacement by a
  package update, and exploit it later.
impact: |
  None known for normal use. Tools that hard link files they do not own (some backup or
  deduplication tools run as a regular user) must run with the needed privileges.
notes: |
  Ubuntu enables it by default, through `/usr/lib/sysctl.d/99-protect-links.conf` (procps) or
  `/usr/lib/sysctl.d/50-default.conf` (systemd), and `/etc/sysctl.d/10-link-restrictions.conf` on 20.04.

setting:
  location: sysctl `fs.protected_hardlinks`, persisted in `/etc/sysctl.d/`
  default_value: "1"
  recommended_value: "1"

audit:
  manual: |
    Run `sysctl fs.protected_hardlinks`. The value must be `1`.
    Then check that no sysctl configuration file sets it to `0`.
  checks:
    - id: running
      engine: sysctl
      description: Running value of fs.protected_hardlinks.
      spec: { key: fs.protected_hardlinks, value: 1 }

remediation:
  manual: |
    Create `/etc/sysctl.d/60-cirsec.conf` containing `fs.protected_hardlinks = 1`,
    remove any other assignment of this key, then run `sysctl --system`.
  effects:
    restart: none
    disruption: none
    lockout_risk: false
  rollback: |
    Remove the line from `/etc/sysctl.d/60-cirsec.conf` and run `sysctl --system`.
  scripts:
    - id: set
      from_check: running

references:
  - title: Ubuntu Security documentation - File handling
    url: https://documentation.ubuntu.com/security/security-features/process-memory/file-handling/
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: Linux kernel documentation - Documentation for /proc/sys/fs/ (protected_hardlinks)
    url: https://docs.kernel.org/admin-guide/sysctl/fs.html
    publisher: The Linux Kernel Organization
    kind: vendor-doc
    accessed: 2026-10-19
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R14)
    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: R14
    relation: partial
    note: R14 lists several filesystem settings; this control covers fs.protected_hardlinks.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-6
    relation: related
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.5.1"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.5.1"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1068
  - framework: cwe
    id: CWE-59

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, BIOS boot
    notes: "Passes on a fresh install (1); still passes after remediation and reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["22.04"]
    environment: Proxmox VM, Ubuntu 22.04 cloud image, kernel 5.15.0-198-generic, BIOS boot
    notes: "Passes on a fresh install (1); still passes after remediation and reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["24.04"]
    environment: Proxmox VM, Ubuntu 24.04 cloud image, kernel 6.8.0-147-generic, BIOS boot
    notes: "Passes on a fresh install (1); still passes after remediation and reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["26.04"]
    environment: Proxmox VM, Ubuntu 26.04 cloud image, kernel 7.0.0-38-generic, BIOS boot
    notes: "Passes on a fresh install (1); still passes after remediation and reboot."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19