# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-BOOT-006
title: Protect symbolic links in world-writable sticky directories
finding:
  title: Symbolic links in shared temporary directories are followed for any user
  description: An attacker can plant a symlink in /tmp and make a privileged program overwrite an arbitrary file.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [kernel, sysctl, filesystem, toctou]

description: |
  With `fs.protected_symlinks = 1`, the kernel only follows a symbolic link located in a
  world-writable sticky directory (such as `/tmp`) when the process following it owns the link,
  or when the link owner also owns the directory.
rationale: |
  A classic attack plants a symbolic link in `/tmp` with the name a privileged program is about
  to create, pointing to a sensitive file. When the program writes "its" file, it overwrites the
  target instead. This setting blocks the whole class of time-of-check to time-of-use races
  based on symlinks in shared directories.
impact: |
  None known for normal use. Programs that rely on following other users' symlinks in `/tmp`
  are rare and already unsafe.
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_symlinks`, persisted in `/etc/sysctl.d/`
  default_value: "1"
  recommended_value: "1"

audit:
  manual: |
    Run `sysctl fs.protected_symlinks`. 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_symlinks.
      spec: { key: fs.protected_symlinks, value: 1 }

remediation:
  manual: |
    Create `/etc/sysctl.d/60-cirsec.conf` containing `fs.protected_symlinks = 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_symlinks)
    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_symlinks.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-6
    relation: related
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.5.2"
    relation: equivalent
    note: The benchmark puts it in its higher level; we apply it everywhere since Ubuntu already does.
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.5.2"
    relation: equivalent
    note: The benchmark puts it in its higher level; we apply it everywhere since Ubuntu already does.
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