# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-UBU-SVC-024
title: Remove the X Window System server and the GNOME display manager from servers
finding:
  title: An X Window System server or a graphical login manager 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, and Xwayland
  on Ubuntu). When it is absent, no X server is installed. `gdm3`, the GNOME display manager of
  Ubuntu Desktop, starts a graphical login screen as soon as it is installed.
rationale: |
  An X server is complex software, historically run with root privileges, with a long record of
  local privilege escalation flaws. A display manager adds a graphical login on the console and
  its own services (accessibility, network manager applets, remote login). A server administered
  over SSH needs neither.
impact: |
  Removes `gdm3`, `xserver-common` and every package that depends on them: X servers, and the
  graphical desktop if one is installed (the `ubuntu-desktop` metapackages go with it). 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' gdm3 xserver-common`. It must not print
    `install ok installed`.
  checks:
    - id: gdm3
      engine: package
      description: GNOME display manager.
      spec: { name: gdm3, state: absent }
    - id: xserver-common
      engine: package
      description: X.Org server common files.
      spec: { name: xserver-common, state: absent }

remediation:
  manual: |
    Run `apt purge gdm3 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: gdm3
      from_check: gdm3
    - id: xserver-common
      from_check: xserver-common

references:
  - title: Xserver(1), Ubuntu manual pages
    url: https://manpages.ubuntu.com/manpages/noble/man1/Xserver.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 (R58)
    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: 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-ubuntu-2004
    framework_version: "3.0.0"
    id: "1.7.1"
    relation: equivalent
  - framework: cis-ubuntu-2004
    framework_version: "3.0.0"
    id: "2.1.20"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "1.7.1"
    relation: equivalent
  - framework: cis-ubuntu-2204
    framework_version: "3.0.0"
    id: "2.1.20"
    relation: equivalent
  - framework: cis-ubuntu-2404
    framework_version: "2.0.0"
    id: "2.1.23"
    relation: equivalent
  - framework: cis-ubuntu-2604
    framework_version: "1.0.0"
    id: "2.1.21"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1068

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; with xvfb installed (xserver-common) the check fails, fix purges it, audit passes after reboot. gdm3 check passes on the stock image (GNOME not installed in this test)."
  - 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; with xvfb installed (xserver-common) the check fails, fix purges it, audit passes after reboot. gdm3 check passes on the stock image (GNOME not installed in this test)."
  - 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; with xvfb installed (xserver-common) the check fails, fix purges it, audit passes after reboot. gdm3 check passes on the stock image (GNOME not installed in this test)."
  - 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; with xvfb installed (xserver-common) the check fails, fix purges it, audit passes after reboot. gdm3 check passes on the stock image (GNOME not installed in this test)."
introduced_in: 0.1.0
created: 2026-10-19
updated: 2026-10-19