Cis Ubuntu1804 V220 1 8 7

---
name: cis-ubuntu1804-v220-1-8-7
description: "Ensure GDM disabling automatic mounting of removable media is not overridden"
category: cis-networking
version: "2.2.0"
author: cyberstrike-official
tags: [cis, ubuntu, linux, ubuntu-18.04, gdm, gnome, automount, lockdown, dconf]
cis_id: "1.8.7"
cis_benchmark: "CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0"
tech_stack: [ubuntu, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# 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       |