# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-BOOT-003
title: Restrict the kernel log buffer to privileged users
finding:
  title: Unprivileged users can read the kernel log buffer
  description: The kernel log can reveal addresses, hardware details and errors useful to an attacker.
status: active
profiles: [core]
risk:
  severity: low
assessment: automated
tags: [kernel, sysctl, information-disclosure]

description: |
  The `kernel.dmesg_restrict` sysctl controls who can read the kernel ring buffer with
  `dmesg` or `/dev/kmsg`. With `1`, only processes with the `CAP_SYSLOG` capability can read it.
rationale: |
  Kernel messages often contain memory addresses, driver and firmware versions, and traces of
  failed operations. Reading them helps a local attacker tune a kernel exploit.
impact: |
  Unprivileged users can no longer run `dmesg`. Administrators use `sudo dmesg` or
  `journalctl -k`, which needs membership of the `adm` or `systemd-journal` group.
notes: |
  Ubuntu kernels are built with this restriction on by default from 22.04 on, so the check
  passes on a fresh install and guards against it being turned off. The Ubuntu 20.04 kernel
  (5.4) is not: the check fails on a fresh 20.04 install and the fix turns it on.

setting:
  location: sysctl `kernel.dmesg_restrict`, persisted in `/etc/sysctl.d/`
  default_value: "1 (0 on 20.04)"
  recommended_value: "1"

audit:
  manual: |
    Run `sysctl kernel.dmesg_restrict`. 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 kernel.dmesg_restrict.
      spec: { key: kernel.dmesg_restrict, value: 1 }

remediation:
  manual: |
    Create `/etc/sysctl.d/60-cirsec.conf` containing `kernel.dmesg_restrict = 1`,
    remove any other assignment of this key, then run `sysctl --system`.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Remove the line from `/etc/sysctl.d/60-cirsec.conf` and run `sysctl -w kernel.dmesg_restrict=0`.
  scripts:
    - id: set
      from_check: running

references:
  - title: Ubuntu Security documentation - Kernel protections
    url: https://documentation.ubuntu.com/security/security-features/kernel-protections/
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: Linux kernel documentation - Documentation for /proc/sys/kernel/ (dmesg_restrict)
    url: https://docs.kernel.org/admin-guide/sysctl/kernel.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 (R9)
    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: R9
    relation: partial
    note: R9 lists several kernel settings; this control covers kernel.dmesg_restrict.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: SI-11
    relation: related
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "1.5.5"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "1.5.5"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1082
  - framework: cwe
    id: CWE-200

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: "Fails on a fresh install (0); generated fix sets 1; kept after reboot; second remediate leaves it alone."
  - 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