# Cis Ubuntu1804 V220 1 8 5 > Ensure GDM screen locks cannot be overridden - Skill: `cyberstrikeus/cis-ubuntu1804-v220-1-8-5` (Agent Skill) - Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-1-8-5` - Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-1-8-5/raw - Safety review: PASS (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/cyberstrikeus/cis-ubuntu1804-v220-1-8-5 --- # 1.8.5 Ensure GDM screen locks cannot be overridden (Automated) ## Profile Applicability - Level 1 - Server - Level 1 - Workstation ## Description GNOME Desktop Manager can make the screen lock automatically whenever the user is idle for some amount of time. By using the lockdown mode in dconf, you can prevent users from changing specific settings. To lock down a dconf key or subpath, create a locks subdirectory in the keyfile directory. The files inside this directory contain a list of keys or subpaths to lock. Just as with the keyfiles, you may add any number of files to this directory. Example Lock File: ``` # Lock desktop screensaver settings /org/gnome/desktop/session/idle-delay /org/gnome/desktop/screensaver/lock-delay ``` ## Rationale Setting a lock-out value reduces the window of opportunity for unauthorized user access to another user's session that has been left unattended. Without locking down the system settings, user settings take precedence over the system settings. ## Audit Procedure ### Command Line Run the following script to verify that the screen lock cannot be overridden: ```bash #!/usr/bin/env bash { # Check if GNOME Desktop Manager is installed. If package isn't installed, recommendation is Not Applicable\n # determine system's package manager l_pkgoutput="" if command -v dpkg-query > /dev/null 2>&1; then l_pq="dpkg-query -s" elif command -v rpm > /dev/null 2>&1; then l_pq="rpm -q" fi # Check if GDM is installed l_pcl="gdm gdm3" # Space separated list of packages to check for l_pn in $l_pcl; do $l_pq "$l_pn" > /dev/null 2>&1 && l_pkgoutput="$l_pkgoutput\n - Package: \"$l_pn\" exists on the system\n - checking configuration" done # Check configuration (If applicable) if [ -n "$l_pkgoutput" ]; then l_output="" l_output2="" # Look for idle-delay to determine profile in use, needed for remaining tests l_kfd="/etc/dconf/db/$(grep -Psril '^\h*idle-delay\h*=\h*uint32\h+\d+\b' /etc/dconf/db/*/ | awk -F'/' '{split($(NF-1),a,".");print a[1]}').d" #set directory of key file to be locked l_kfd2="/etc/dconf/db/$(grep -Psril '^\h*lock-delay\h*=\h*uint32\h+\d+\b' /etc/dconf/db/*/ | awk -F'/' '{split($(NF-1),a,".");print a[1]}').d" #set directory of key file to be locked if [ -d "$l_kfd" ]; then # If key file directory doesn't exist, options can't be locked if grep -Prilq '\/org\/gnome\/desktop\/session\/idle-delay\b' "$l_kfd"; then l_output="$l_output\n - \"idle-delay\" is locked in \"$(grep -Pril '\/org\/gnome\/desktop\/session\/idle-delay\b' "$l_kfd")\"" else l_output2="$l_output2\n - \"idle-delay\" is not locked" fi else l_output2="$l_output2\n - \"idle-delay\" is not set so it can not be locked" fi if [ -d "$l_kfd2" ]; then # If key file directory doesn't exist, options can't be locked if grep -Prilq '\/org\/gnome\/desktop\/screensaver\/lock-delay\b' "$l_kfd2"; then l_output="$l_output\n - \"lock-delay\" is locked in \"$(grep -Pril '\/org\/gnome\/desktop\/screensaver\/lock-delay\b' "$l_kfd2")\"" else l_output2="$l_output2\n - \"lock-delay\" is not locked" fi else l_output2="$l_output2\n - \"lock-delay\" is not set so it can not be locked" fi else l_output="$l_output\n - GNOME Desktop Manager package is not installed on the system\n - Recommendation is not applicable" fi # Report results. If no failures output in l_output2, we pass [ -n "$l_pkgoutput" ] && echo -e "\n$l_pkgoutput" if [ -z "$l_output2" ]; then echo -e "\n- Audit Result:\n ** PASS **\n$l_output\n" else echo -e "\n- Audit Result:\n ** FAIL **\n - Reason(s) for audit failure:\n$l_output2\n" [ -n "$l_output" ] && echo -e "\n- Correctly set:\n$l_output\n" fi } ``` ## Remediation ### Command Line Run the following script to ensure screen locks can not be overridden: ```bash #!/usr/bin/env bash { # Check if GNOME Desktop Manager is installed. If package isn't installed, recommendation is Not Applicable\n # determine system's package manager l_pkgoutput="" if command -v dpkg-query > /dev/null 2>&1; then l_pq="dpkg-query -s" elif command -v rpm > /dev/null 2>&1; then l_pq="rpm -q" fi # Check if GDM is installed l_pcl="gdm gdm3" # Space separated list of packages to check for l_pn in $l_pcl; do $l_pq "$l_pn" > /dev/null 2>&1 && l_pkgoutput="$l_pkgoutput\n - Package: \"$l_pn\" exists on the system\n - remediating configuration if needed" done # Check configuration (If applicable) if [ -n "$l_pkgoutput" ]; then # Look for idle-delay to determine profile in use, needed for remaining tests l_kfd="/etc/dconf/db/$(grep -Psril '^\h*idle-delay\h*=\h*uint32\h+\d+\b' /etc/dconf/db/*/ | awk -F'/' '{split($(NF-1),a,".");print a[1]}').d" #set directory of key file to be locked # Look for lock-delay to determine profile in use, needed for remaining tests l_kfd2="/etc/dconf/db/$(grep -Psril '^\h*lock-delay\h*=\h*uint32\h+\d+\b' /etc/dconf/db/*/ | awk -F'/' '{split($(NF-1),a,".");print a[1]}').d" #set directory of key file to be locked if [ -d "$l_kfd" ]; then # If key file directory doesn't exist, options can't be locked if grep -Prilq '^\h*\/org\/gnome\/desktop\/session\/idle-delay\b' "$l_kfd"; then echo " - \"idle-delay\" is locked in \"$(grep -Pril '^\h*\/org\/gnome\/desktop\/session\/idle-delay\b' "$l_kfd")\"" else echo " - creating entry to lock \"idle-delay\"" { ! -d "$l_kfd"/locks } && echo "creating directory $l_kfd/locks" && mkdir "$l_kfd"/locks { echo -e '\n# Lock desktop screensaver idle-delay setting' echo '/org/gnome/desktop/session/idle-delay' } >> "$l_kfd"/locks/00-screensaver fi else echo -e " - \"idle-delay\" is not set so it can not be locked\n - Please follow Recommendation \"Ensure GDM screen locks when the user is idle\" and follow this Recommendation again" fi if [ -d "$l_kfd2" ]; then # If key file directory doesn't exist, options can't be locked if grep -Prilq '^\h*\/org\/gnome\/desktop\/screensaver\/lock-delay\b' "$l_kfd2"; then echo " - \"lock-delay\" is locked in \"$(grep -Pril '^\h*\/org\/gnome\/desktop\/screensaver\/lock-delay\b' "$l_kfd2")\"" else echo " - creating entry to lock \"lock-delay\"" { ! -d "$l_kfd2"/locks } && echo "creating directory $l_kfd2/locks" && mkdir "$l_kfd2"/locks { echo -e '\n# Lock desktop screensaver lock-delay setting' echo '/org/gnome/desktop/screensaver/lock-delay' } >> "$l_kfd2"/locks/00-screensaver fi else echo -e " - \"lock-delay\" is not set so it can not be locked\n - Please follow Recommendation \"Ensure GDM screen locks when the user is idle\" and follow this Recommendation again" fi else echo -e " - GNOME Desktop Manager package is not installed on the system\n - Recommendation is not applicable" fi } ``` Run the following command to update the system databases: ```bash dconf update ``` Note: Users must log out and back in again before the system-wide settings take effect. ## References 1. https://help.gnome.org/admin/system-admin-guide/stable/desktop-lockscreen.html.en 2. https://help.gnome.org/admin/system-admin-guide/stable/dconf-lockdown.html.en 3. NIST SP 800-53 Rev. 5: CM-11 ## CIS Controls | Controls Version | Control | IG 1 | IG 2 | IG 3 | | ---------------- | ------------------------------------------------------------ | ---- | ---- | ---- | | v8 | 4.3 Configure Automatic Session Locking on Enterprise Assets | X | X | X | | v7 | 16.11 Lock Workstation Sessions After Inactivity | X | X | X | ## MITRE ATT&CK Mappings | Techniques / Sub-techniques | Tactics | Mitigations | | --------------------------- | ------- | ----------- | | T1456 | TA0027 | M1001 |