# Cis Ubuntu1804 V220 1 8 7 > Ensure GDM disabling automatic mounting of removable media is not overridden - Skill: `cyberstrikeus/cis-ubuntu1804-v220-1-8-7` (Agent Skill) - Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-1-8-7` - Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-1-8-7/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-v220-1-8-7 --- # 1.8.7 Ensure GDM disabling automatic mounting of removable media is not overridden (Automated) ## Profile Applicability - Level 1 - Server - Level 2 - Workstation ## Description By default GNOME automatically mounts removable media when inserted as a convenience to the user. 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 media-handling settings /org/gnome/desktop/media-handling/automount /org/gnome/desktop/media-handling/automount-open ``` ## Rationale With automounting enabled anyone with physical access could attach a USB drive or disc and have its contents available in system even if they lacked permissions to mount it themselves. ## Impact The use of portable hard drives is very common for workstation users. ## Audit Procedure ### Command Line Run the following script to verify disable automatic mounting is locked: ```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 "$l_pkgoutput" ]; then l_output="" l_output2="" # Look for automount to determine profile in use, needed for remaining tests l_kfd="/etc/dconf/db/$(grep -Psril '^\h*automount\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*automount-open\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 -Piq '^\h*\/org\/gnome\/desktop\/media-handling\/automount\b' "$l_kfd"; then l_output="$l_output\n - \"automount\" is locked in \"$(grep -Pril '^\h*\/org\/gnome\/desktop\/media-handling\/automount\b' "$l_kfd")\"" else l_output2="$l_output2\n - \"automount\" is not locked" fi else l_output2="$l_output2\n - \"automount\" is not set so it can not be locked" fi if [ -d "$l_kfd2" ]; then # If key file directory doesn't exist, options can't be locked if grep -Piq '^\h*\/org\/gnome\/desktop\/media-handling\/automount-open\b' "$l_kfd2"; then l_output="$l_output\n - \"automount-open\" is locked in \"$(grep -Pril '^\h*\/org\/gnome\/desktop\/media-handling\/automount-open\b' "$l_kfd2")\"" else l_output2="$l_output2\n - \"automount-open\" is not locked" fi else l_output2="$l_output2\n - \"automount-open\" is not set so it can not be locked" fi else l_output="$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 lock disable automatic mounting of media for all GNOME users: ```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 "$l_pkgoutput" ]; then # Look for automount to determine profile in use, needed for remaining tests l_kfd="/etc/dconf/db/$(grep -Psril '^\h*automount\b' /etc/dconf/db/*/ | awk -F'/' '{split($(NF-1),a,".");print a[1]}').d" #set directory of key file to be locked # Look for automount-open to determine profile in use, needed for remaining tests l_kfd2="/etc/dconf/db/$(grep -Psril '^\h*automount-open\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 -Priq '^\h*\/org\/gnome\/desktop\/media-handling\/automount\b' "$l_kfd"; then echo " - \"automount\" is locked in \"$(grep -Pril '^\h*\/org\/gnome\/desktop\/media-handling\/automount\b' "$l_kfd")\"" else echo " - creating entry to lock \"automount\"" { ! -d "$l_kfd"/locks } && echo "creating directory $l_kfd/locks" && mkdir "$l_kfd"/locks { echo -e '\n# Lock desktop media-handling automount setting' echo '/org/gnome/desktop/media-handling/automount' } >> "$l_kfd"/locks/00-media-automount fi else echo -e " - \"automount\" is not set so it can not be locked\n - Please follow Recommendation \"Ensure GDM automatic mounting of removable media is disabled\" and follow this Recommendation again" fi if [ -d "$l_kfd2" ]; then # If key file directory doesn't exist, options can't be locked if grep -Priq '^\h*\/org\/gnome\/desktop\/media-handling\/automount-open\b' "$l_kfd2"; then echo " - \"automount-open\" is locked in \"$(grep -Pril '^\h*\/org\/gnome\/desktop\/media-handling\/automount-open\b' "$l_kfd2")\"" else echo " - creating entry to lock \"automount-open\"" { ! -d "$l_kfd2"/locks } && echo "creating directory $l_kfd2/locks" && mkdir "$l_kfd2"/locks { echo -e '\n# Lock desktop media-handling automount-open setting' echo '/org/gnome/desktop/media-handling/automount-open' } >> "$l_kfd2"/locks/00-media-automount fi else echo -e " - \"automount-open\" is not set so it can not be locked\n - Please follow Recommendation \"Ensure GDM automatic mounting of removable media is disabled\" and follow this Recommendation again" fi # update dconf database dconf update else echo -e " - GNOME Desktop Manager package is not installed on the system\n - Recommendation is not applicable" fi } ``` ## References 1. https://help.gnome.org/admin/system-admin-guide/stable/dconf-lockdown.html.en 2. NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5 ## MITRE ATT&CK Mappings | Techniques / Sub-techniques | Tactics | Mitigations | | --------------------------- | -------------- | ----------- | | T1091, T1091.000 | TA0001, TA0008 | M1042 |