cis-ubuntu2004-v300-1-7-7

name	cis-ubuntu2004-v300-1-7-7
description	Ensure GDM disabling automatic mounting of removable media is not overridden
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","gnome","gdm","removable-media","dconf-lock"]
cis_id	1.7.7
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

1.7.7 Ensure GDM disabling automatic mounting of removable media is not overridden (Automated)

Profile

• 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.

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 command to verify automount=false and automount-open=false is correctly configured to ensure automatic mounting of removable media is not overridden:

# grep -Psi "automount" /etc/dconf/db/*/locks/*

/etc/dconf/db/local.d/locks/00-media-automount:automount=false
/etc/dconf/db/local.d/locks/00-media-automount:automount-open=false

Expected Result

Lock entries for automount and automount-open should be present.

Remediation

Command Line

1. To prevent the user from overriding these settings, create the file /etc/dconf/db/local.d/locks/00-media-automount with the following content:

[org/gnome/desktop/media-handling]
automount=false
automount-open=false

2. Update the systems databases:

# dconf update

Note:

• A user profile must exist in order to apply locks. If a user profile does not exist review the remediation steps in the previous recommendation.
• 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/dconf-lockdown.html.en
2. NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5