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:

#!/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:

#!/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