# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-NET-014
title: Ignore IPv6 router advertisements
finding:
  title: The kernel configures IPv6 addresses and routes from any router advertisement
  description: Anyone on the local network can announce themselves as an IPv6 router and receive the host's traffic.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [sysctl, network, ipv6]

applicability:
  facts: [ipv6]

description: |
  IPv6 routers announce prefixes and default routes with router advertisements (RA). With
  `accept_ra` on, the kernel uses them to add addresses and a default route on its own. This
  control sets `net.ipv6.conf.all.accept_ra` and `net.ipv6.conf.default.accept_ra` (interfaces
  created later) to `0`, so that IPv6 addresses and routes are only those configured on purpose.
rationale: |
  Router advertisements are not authenticated. A rogue RA, sent by an attacker or by a
  misconfigured machine, makes the host route its IPv6 traffic, often preferred over IPv4,
  through the sender (man in the middle) or lose connectivity.
impact: |
  Hosts that get their IPv6 address and route from SLAAC lose them for interfaces brought up after
  the change. Configure IPv6 statically, or let the network manager handle RAs on the trusted
  interfaces only (systemd-networkd `IPv6AcceptRA=`, NetworkManager `ipv6.method`).
notes: |
  systemd-networkd processes RAs itself and keeps the kernel `accept_ra` of the links it manages
  at `0`, so this setting does not prevent networkd from using RAs when its configuration asks
  for it. Disabling IPv6 altogether is decided in CB-UBU-NET-001.

setting:
  location: sysctl `net.ipv6.conf.{all,default}.accept_ra`, persisted in `/etc/sysctl.d/`
  default_value: "1"
  recommended_value: "0"

audit:
  manual: |
    Run `sysctl net.ipv6.conf.all.accept_ra net.ipv6.conf.default.accept_ra`. Both must be `0`.
    Check that no sysctl file sets another value.
  checks:
    - id: all-accept-ra
      engine: sysctl
      description: net.ipv6.conf.all.accept_ra.
      spec: { key: net.ipv6.conf.all.accept_ra, value: 0 }
    - id: default-accept-ra
      engine: sysctl
      description: net.ipv6.conf.default.accept_ra.
      spec: { key: net.ipv6.conf.default.accept_ra, value: 0 }

remediation:
  manual: |
    Add to `/etc/sysctl.d/60-cirsec.conf`:
    ```
    net.ipv6.conf.all.accept_ra = 0
    net.ipv6.conf.default.accept_ra = 0
    ```
    remove other assignments, then run `sysctl --system`.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Remove the lines from `/etc/sysctl.d/60-cirsec.conf` and run `sysctl -w net.ipv6.conf.all.accept_ra=1 net.ipv6.conf.default.accept_ra=1`.
  scripts:
    - id: all-accept-ra
      from_check: all-accept-ra
    - id: default-accept-ra
      from_check: default-accept-ra

references:
  - title: Linux kernel documentation - IP Sysctl
    url: https://docs.kernel.org/networking/ip-sysctl.html
    publisher: The Linux Kernel Organization
    kind: vendor-doc
    accessed: 2026-10-19
  - title: RFC 4861 - Neighbor Discovery for IP version 6 (IPv6)
    url: https://www.rfc-editor.org/rfc/rfc4861
    publisher: IETF
    kind: rfc
    accessed: 2026-10-19
mappings:
  - framework: anssi-linux
    framework_version: "2.0"
    id: R13
    relation: related
    note: R13 asks to disable IPv6 when unused, and to secure it otherwise.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7
    relation: related
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "3.3.11"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "3.3.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "3.3.2.8"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "3.3.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "3.3.2.8"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "3.3.2.7"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "3.3.2.8"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1557
  - framework: cwe
    id: CWE-290
related_controls:
  - id: CB-UBU-NET-001
    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, BIOS boot
    notes: "accept_ra 1 reported; generated fixes; kept after reboot; DHCP connectivity through networkd unaffected."
  - 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: "accept_ra 1 reported; generated fixes; kept after reboot; DHCP connectivity through networkd unaffected."
  - 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: "accept_ra 1 reported; generated fixes; kept after reboot; DHCP connectivity through networkd unaffected."
  - 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: "accept_ra 1 reported; generated fixes; kept after reboot; DHCP connectivity through networkd unaffected."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19