# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-SVC-024
title: Remove the X Window System server from servers
finding:
  title: An X Window System server is installed on a server
  description: A graphical server stack adds a large amount of privileged, rarely needed code.
status: active
profiles: [server-hardened]
risk:
  severity: low
assessment: automated
tags: [packages, x11, attack-surface]

description: |
  `xserver-common` holds the files shared by every X.Org server (Xorg, Xvfb, Xephyr...). When it
  is absent, no X server is installed.
rationale: |
  An X server is complex software, historically run with root privileges, with a long record of
  local privilege escalation flaws. A server administered over SSH does not need it.
impact: |
  Removes `xserver-common` and every package that depends on it: X servers, and the graphical
  desktop if one is installed. X client applications forwarded over SSH (`ssh -X`) still work,
  since they use the X server of the administrator's workstation.
notes: |
  The control is only in the hardened server profile; desktops need an X server or Xwayland.

audit:
  manual: |
    Run `dpkg-query -W -f='${Status}\n' xserver-common`. It must not print `install ok installed`.
  checks:
    - id: xserver-common
      engine: package
      description: X.Org server common files.
      spec: { name: xserver-common, state: absent }

remediation:
  manual: |
    Run `apt purge xserver-common` after checking the list of packages apt will remove.
  effects:
    restart: none
    disruption: low
    lockout_risk: false
  rollback: |
    Reinstall the needed X server, for example `apt install xserver-xorg-core`.
  scripts:
    - id: xserver-common
      from_check: xserver-common

references:
  - title: Xserver(1), Debian manual pages
    url: https://manpages.debian.org/trixie/xserver-common/Xserver.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 (R58)
    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: R58
    relation: partial
    note: R58 asks to install only the packages that are strictly needed; this control covers the X server.
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7
    relation: partial
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "2.1.23"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "2.1.21"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1068

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 xserver-common installed the check fails, fix purges it, audit passes after reboot; SSH reachable."
  - 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 xserver-common installed the check fails, fix purges it, audit passes after reboot; SSH reachable."
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18