# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-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 Debian.

  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), Debian manual pages
    url: https://manpages.debian.org/trixie/squid/squid.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: systemctl(1), Debian manual pages
    url: https://manpages.debian.org/trixie/systemd/systemctl.1.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - 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: 2026-10-18
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-debian-12
    framework_version: "2.0.0"
    id: "2.1.21"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "2.1.18"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1090
related_controls:
  - id: CB-DEB-SVC-008
    type: related

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["12"]
    environment: Proxmox VM, Debian 12 genericcloud image, kernel 6.1.0-54-cloud-amd64
    notes: "Fresh image passes; with squid installed the check fails, fix masks, audit passes after reboot."
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["13"]
    environment: Proxmox VM, Debian 13 genericcloud image, kernel 6.12.113+deb13-cloud-amd64
    notes: "Fresh image passes; with squid installed the check fails, fix masks, audit passes after reboot."
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18