# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-SVC-009
title: Disable the Squid web proxy on systems that are not proxy servers
finding:
  title: A web proxy can run on a system that is not a proxy server
  description: An open or forgotten proxy relays traffic for others and hides its origin.
status: active
profiles: [core]
risk:
  severity: medium
assessment: automated
tags: [services, proxy, http, attack-surface]

description: |
  Squid (`squid.service`) is the HTTP caching proxy packaged by Ubuntu.

  The control passes when the unit is masked or not installed at all.
rationale: |
  A proxy relays connections for its clients. A misconfigured one lets outsiders reach internal
  systems or use the host to hide attacks, and its cache can hold sensitive responses.
impact: |
  Masking the unit stops it and prevents it from starting again. Proxy servers must exclude this
  control in their tailoring file.
notes: |
  A unit that does not exist counts as compliant, so removing the package is an equally valid fix.

audit:
  manual: |
    Run `systemctl is-enabled squid.service`. It must print `masked`, or fail because the unit does
    not exist.
  checks:
    - id: squid
      engine: systemd-unit
      description: Squid proxy server.
      spec: { unit: squid.service, state: masked }

remediation:
  manual: |
    Run `systemctl mask --now squid.service`, or `apt purge squid` if the package is not needed.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Run `systemctl unmask squid.service` then `systemctl enable --now squid.service`.
  scripts:
    - id: squid
      from_check: squid

references:
  - title: squid(8), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man8/squid.8.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: systemctl(1), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man1/systemctl.1.html
    publisher: Canonical
    kind: vendor-doc
    accessed: 2026-10-19
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R62)
    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: R62
    relation: partial
    note: R62 asks to disable every unneeded service; this control covers the web proxy.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7
    relation: partial
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "2.1.17"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "2.1.17"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "2.1.21"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "2.1.18"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1090
related_controls:
  - id: CB-UBU-SVC-008
    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: "Stock image passes (units absent); with squid installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["22.04"]
    environment: Proxmox VM, Ubuntu 22.04 cloud image, kernel 5.15.0-198-generic
    notes: "Stock image passes (units absent); with squid installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["24.04"]
    environment: Proxmox VM, Ubuntu 24.04 cloud image, kernel 6.8.0-147-generic
    notes: "Stock image passes (units absent); with squid installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
  - date: 2026-10-19
    by: Sn0wAlice
    versions: ["26.04"]
    environment: Proxmox VM, Ubuntu 26.04 cloud image, kernel 7.0.0-38-generic
    notes: "Stock image passes (units absent); with squid installed the check fails, server-hardened remediation masks the units, audit passes, unchanged on a second run and after reboot."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19