Require a password at the graphical login, without automatic login or guest session

Description

Display managers can open a session without authentication:

• GDM: AutomaticLoginEnable and TimedLoginEnable in the [daemon] section of /etc/gdm3/custom.conf;
• LightDM: autologin-user, autologin-guest and the guest session (allow-guest, on by default upstream);
• SDDM: User in the [Autologin] section of its configuration.

This control checks that none of them is enabled and that LightDM explicitly disables the guest session.

Ubuntu's GDM reads /etc/gdm3/custom.conf (not daemon.conf as on Debian). The Ubuntu installer writes AutomaticLoginEnable=true there when "Log in automatically" is chosen. GDM has no guest session. Whether LightDM's guest session works depends on the guest helper installed, so the option is turned off explicitly. The LightDM and SDDM checks read every configuration file in load order and keep the last value; they ignore sections, so a value set for one seat only counts for all. The automated fix covers GDM only; the LightDM and SDDM checks were written from their documentation and not tested in a lab.

Rationale

A session opened without a password gives the account's files, browser sessions, stored credentials and network access to whoever is in front of the screen, or to whoever steals the machine. A guest session gives an unaudited foothold on the system.

Impact

Every user types a password at boot. Kiosks that rely on automatic login must be excluded through tailoring.

Setting

Audit

1. GDM: /etc/gdm3/custom.conf must not set AutomaticLoginEnable=true or TimedLoginEnable=true in [daemon].
2. LightDM: the configuration must set allow-guest=false, and must not set autologin-user or autologin-guest=true.
3. SDDM: no User= value in an [Autologin] section of /etc/sddm.conf, /etc/sddm.conf.d/ or /usr/lib/sddm/sddm.conf.d/.

Automated: every check must pass. Scanners report each check by its id.

gdm

GDM has no automatic or timed login.

script: |
  [ -f /etc/gdm3/custom.conf ] || { echo "no /etc/gdm3/custom.conf"; exit 0; }
  rc=0
  for k in AutomaticLoginEnable TimedLoginEnable; do
    v=$(K=$k awk '/^[ \t]*\[/ { h = $0; gsub(/^[ \t]+|[ \t]+$/, "", h); s = (h == "[daemon]"); next }
      s && $0 ~ "^[ \t]*" ENVIRON["K"] "[ \t]*=" { v = $0; sub(/^[^=]*=[ \t]*/, "", v); sub(/[ \t]+$/, "", v) }
      END { print v }' /etc/gdm3/custom.conf)
    echo "$k=${v:-unset}"
    case $v in [Tt][Rr][Uu][Ee] | 1) rc=1 ;; esac
  done
  exit "$rc"

lightdm

LightDM has no automatic login and no guest session.

script: |
  files=
  for f in /usr/share/lightdm/lightdm.conf.d/*.conf /etc/xdg/lightdm/lightdm.conf.d/*.conf \
    /etc/lightdm/lightdm.conf.d/*.conf /etc/lightdm/lightdm.conf; do
    [ -f "$f" ] && files="$files $f"
  done
  get() {
    # shellcheck disable=SC2086 # one word per file
    [ -n "$files" ] && K=$1 awk '$0 ~ "^[ \t]*" ENVIRON["K"] "[ \t]*=" { v = $0; sub(/^[^=]*=[ \t]*/, "", v); sub(/[ \t]+$/, "", v) }
      END { print v }' $files
  }
  g=$(get allow-guest) u=$(get autologin-user) a=$(get autologin-guest)
  echo "allow-guest=${g:-unset} autologin-user=${u:-unset} autologin-guest=${a:-unset}"
  case $g in [Ff][Aa][Ll][Ss][Ee] | 0) ;; *) exit 1 ;; esac
  [ -z "$u" ] || exit 1
  case $a in [Tt][Rr][Uu][Ee] | 1) exit 1 ;; esac

sddm

SDDM has no automatic login.

script: |
  files=
  for f in /usr/lib/sddm/sddm.conf.d/*.conf /etc/sddm.conf.d/*.conf /etc/sddm.conf; do
    [ -f "$f" ] && files="$files $f"
  done
  [ -n "$files" ] || { echo "no SDDM configuration file"; exit 0; }
  # shellcheck disable=SC2086 # one word per file
  u=$(awk 'FNR == 1 { s = 0 } /^[ \t]*\[/ { h = $0; gsub(/^[ \t]+|[ \t]+$/, "", h); s = (h == "[Autologin]"); next }
    s && /^[ \t]*User[ \t]*=/ { v = $0; sub(/^[^=]*=[ \t]*/, "", v); sub(/[ \t]+$/, "", v) }
    END { print v }' $files)
  echo "[Autologin] User=${u:-unset}"
  [ -z "$u" ]

Remediation

• GDM: in /etc/gdm3/custom.conf, section [daemon], set AutomaticLoginEnable=false and TimedLoginEnable=false (or remove the lines). The change applies when GDM restarts.
• LightDM: in /etc/lightdm/lightdm.conf, section [Seat:*], set allow-guest=false, greeter-allow-guest=false, and remove autologin-user and autologin-guest.
• SDDM: remove User= from the [Autologin] section of the SDDM configuration files.

Rollback

Restore /etc/gdm3/custom.conf from the backup directory.

gdm

# Turn off automatic and timed login in GDM. Only edits /etc/gdm3/custom.conf when one is on.
f=/etc/gdm3/custom.conf
[ -f "$f" ] || { echo "$f not found" >&2; exit 1; }

# ini_get SECTION KEY: last value of KEY in [SECTION] of custom.conf, empty when unset.
ini_get() {
  S="[$1]" K=$2 awk '
    BEGIN { s = ENVIRON["S"]; k = ENVIRON["K"] }
    /^[ \t]*\[/ { h = $0; gsub(/^[ \t]+|[ \t]+$/, "", h); in_s = (h == s); next }
    in_s && /=/ && !/^[ \t]*[#;]/ {
      key = substr($0, 1, index($0, "=") - 1); gsub(/^[ \t]+|[ \t]+$/, "", key)
      if (key == k) { v = substr($0, index($0, "=") + 1); gsub(/^[ \t]+|[ \t]+$/, "", v) }
    }
    END { print v }
  ' "$f"
}

# ini_set SECTION KEY VALUE: set KEY=VALUE right after the [SECTION] header, adding the section if missing.
ini_set() {
  S="[$1]" K=$2 V=$3 awk '
    BEGIN { s = ENVIRON["S"]; k = ENVIRON["K"]; v = ENVIRON["V"] }
    function out() { if (!done) { print k "=" v; done = 1 } }
    /^[ \t]*\[/ { print; h = $0; gsub(/^[ \t]+|[ \t]+$/, "", h); in_s = (h == s); if (in_s) out(); next }
    in_s && $0 ~ "^[ \t]*" k "[ \t]*=" { next }
    { print }
    END { if (!done) { print s; out() } }
  ' "$f" >"$f.cirsec-tmp" && cat "$f.cirsec-tmp" >"$f"
  rc=$?
  rm -f "$f.cirsec-tmp"
  return "$rc"
}

is_true() { case $1 in [Tt][Rr][Uu][Ee] | 1) return 0 ;; esac; return 1; }

for k in AutomaticLoginEnable TimedLoginEnable; do
  is_true "$(ini_get daemon "$k")" || continue
  cb_backup "$f"
  ini_set daemon "$k" false || exit 1
done

Handwritten code reads parameters from CB_<NAME> environment variables. Each release turns these fixes into remediate.sh and an Ansible role per profile: dry run by default, a fix runs only when its check fails.

References

• GNOME System Administration Guide - Log in automatically The GNOME Project, vendor-doc, accessed 2026-10-19
• sddm.conf(5), Ubuntu manual pages Canonical, vendor-doc, accessed 2026-10-19

Mappings

How this control relates to the other framework's: equivalent, stricter, looser, partial or related.

Threats mitigated

People and lifecycle