# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-DESK-005
title: Disable XDMCP in the display manager
finding:
  title: The display manager accepts XDMCP remote logins
  description: XDMCP sends the login screen, passwords and the whole session unencrypted over the network.
status: active
profiles: [desktop]
risk:
  severity: high
assessment: automated
tags: [gdm, lightdm, xdmcp, network]

description: |
  XDMCP (X Display Manager Control Protocol) lets a remote X terminal request a login screen
  from the display manager. It is off by default in GDM (`[xdmcp] Enable` in
  `/etc/gdm3/daemon.conf`) and in LightDM (`[XDMCPServer] enabled`). This control checks that
  nobody turned it on.
rationale: |
  XDMCP and the X11 session it opens are not encrypted or authenticated: anyone on the network
  path can read the password typed at login and every keystroke after. Remote graphical access
  should go through an encrypted channel such as SSH or a VPN.
impact: |
  None with the defaults. Thin clients that rely on XDMCP stop working.
notes: |
  SDDM has no XDMCP server. The LightDM check reads `/usr/share/lightdm/lightdm.conf.d/`,
  `/etc/xdg/lightdm/lightdm.conf.d/`, `/etc/lightdm/lightdm.conf.d/` and
  `/etc/lightdm/lightdm.conf` in that order; the last value wins. The automated fix covers GDM
  only; the LightDM check was written from its documentation and not tested in a lab.

setting:
  location: /etc/gdm3/daemon.conf [xdmcp] Enable; LightDM [XDMCPServer] enabled
  default_value: "false"
  recommended_value: false or unset

audit:
  manual: |
    1. GDM: in `/etc/gdm3/daemon.conf`, the `[xdmcp]` section must not set `Enable=true`.
    2. LightDM: no file of `/etc/lightdm/lightdm.conf.d/` or `/etc/lightdm/lightdm.conf` may set
       `enabled=true` in an `[XDMCPServer]` section.
  checks:
    - id: gdm
      engine: shell
      description: GDM does not enable XDMCP.
      facts: [gdm]
      spec:
        script: |
          [ -f /etc/gdm3/daemon.conf ] || { echo "no /etc/gdm3/daemon.conf"; exit 0; }
          v=$(awk '/^[ \t]*\[/ { h = $0; gsub(/^[ \t]+|[ \t]+$/, "", h); s = (h == "[xdmcp]"); next }
            s && /^[ \t]*Enable[ \t]*=/ { v = $0; sub(/^[^=]*=[ \t]*/, "", v); sub(/[ \t]+$/, "", v) }
            END { print v }' /etc/gdm3/daemon.conf)
          echo "[xdmcp] Enable=${v:-unset}"
          case $v in [Tt][Rr][Uu][Ee] | 1) exit 1 ;; esac
    - id: lightdm
      engine: shell
      description: LightDM does not enable its XDMCP server.
      facts: [lightdm]
      spec:
        script: |
          set -- /usr/share/lightdm/lightdm.conf.d/*.conf /etc/xdg/lightdm/lightdm.conf.d/*.conf \
            /etc/lightdm/lightdm.conf.d/*.conf /etc/lightdm/lightdm.conf
          files=
          for f in "$@"; do [ -f "$f" ] && files="$files $f"; done
          [ -n "$files" ] || { echo "no LightDM configuration file"; exit 0; }
          # shellcheck disable=SC2086 # one word per file
          v=$(awk 'FNR == 1 { s = 0 } /^[ \t]*\[/ { h = $0; gsub(/^[ \t]+|[ \t]+$/, "", h); s = (h == "[XDMCPServer]"); next }
            s && /^[ \t]*enabled[ \t]*=/ { v = $0; sub(/^[^=]*=[ \t]*/, "", v); sub(/[ \t]+$/, "", v) }
            END { print v }' $files)
          echo "[XDMCPServer] enabled=${v:-unset}"
          case $v in [Tt][Rr][Uu][Ee] | 1) exit 1 ;; esac

remediation:
  manual: |
    - GDM: in `/etc/gdm3/daemon.conf`, set `Enable=false` in the `[xdmcp]` section (or remove the
      line), then restart GDM at a convenient time (`systemctl restart gdm` closes every
      graphical session).
    - LightDM: remove `enabled=true` from the `[XDMCPServer]` sections, then restart LightDM.
  effects:
    restart: reboot
    disruption: low
    lockout_risk: false
  rollback: |
    Restore `/etc/gdm3/daemon.conf` from the backup directory.
  scripts:
    - id: gdm
      engine: shell
      facts: [gdm]
      file: fix.sh

references:
  - title: gdm3(8), Debian manual pages
    url: https://manpages.debian.org/trixie/gdm3/gdm3.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: GDM - configuration keys (gdm.schemas)
    url: https://gitlab.gnome.org/GNOME/gdm/-/blob/main/data/gdm.schemas.in
    publisher: The GNOME Project
    kind: vendor-doc
    accessed: 2026-10-18
  - title: Debian Wiki - LightDM
    url: https://wiki.debian.org/LightDM
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
mappings:
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "1.7.6"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "1.7.6"
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7
    relation: related
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: SC-8
    relation: related
threats:
  - framework: mitre-attack
    id: T1040
  - framework: cwe
    id: CWE-319

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["12"]
    environment: Proxmox VM with minimal GNOME (gdm3, gnome-shell), Debian 12 genericcloud image, kernel 6.1.0-54-cloud-amd64
    notes: "GDM check and fix tested (desktop-hardened), including Enable=true in [xdmcp] fixed without touching [debug]; the LightDM check was not tested (no LightDM machine), only reported not applicable."
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["13"]
    environment: Proxmox VM with minimal GNOME (gdm3, gnome-shell), Debian 13 genericcloud image, kernel 6.12.113+deb13-cloud-amd64
    notes: "GDM check and fix tested (desktop-hardened), including Enable=true in [xdmcp] fixed without touching [debug]; the LightDM check was not tested (no LightDM machine), only reported not applicable."
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18