Disable Mounting of squashfs

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Description

To configure the system to prevent the squashfs kernel module from being loaded, add the following line to the file /etc/modprobe.d/squashfs.conf:

install squashfs /bin/true

This effectively prevents usage of this uncommon filesystem.

The squashfs filesystem type is a compressed read-only Linux filesystem embedded in small footprint systems (similar to cramfs). A squashfs image can be used without having to first decompress the image.

Rationale

Removing support for unneeded filesystem types reduces the local attack surface of the system.

Remediation

Shell script

The following script can be run on the host to remediate the issue.

#!/bin/bash

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

if LC_ALL=C grep -q -m 1 "^install squashfs" /etc/modprobe.d/squashfs.conf ; then
	
	sed -i 's#^install squashfs.*#install squashfs /bin/false#g' /etc/modprobe.d/squashfs.conf
else
	echo -e "\n# Disable per security requirements" >> /etc/modprobe.d/squashfs.conf
	echo "install squashfs /bin/false" >> /etc/modprobe.d/squashfs.conf
fi

if ! LC_ALL=C grep -q -m 1 "^blacklist squashfs$" /etc/modprobe.d/squashfs.conf ; then
	echo "blacklist squashfs" >> /etc/modprobe.d/squashfs.conf
fi

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi

Ansible playbook

The following playbook can be run with Ansible to remediate the issue.

- name: Ensure kernel module 'squashfs' is disabled
  lineinfile:
    create: true
    dest: /etc/modprobe.d/squashfs.conf
    regexp: install\s+squashfs
    line: install squashfs /bin/false
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-83855-7
  - NIST-800-171-3.4.6
  - NIST-800-53-CM-6(a)
  - NIST-800-53-CM-7(a)
  - NIST-800-53-CM-7(b)
  - disable_strategy
  - kernel_module_squashfs_disabled
  - low_complexity
  - low_severity
  - medium_disruption
  - reboot_required

- name: Ensure kernel module 'squashfs' is blacklisted
  lineinfile:
    create: true
    dest: /etc/modprobe.d/squashfs.conf
    regexp: ^blacklist squashfs$
    line: blacklist squashfs
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-83855-7
  - NIST-800-171-3.4.6
  - NIST-800-53-CM-6(a)
  - NIST-800-53-CM-7(a)
  - NIST-800-53-CM-7(b)
  - disable_strategy
  - kernel_module_squashfs_disabled
  - low_complexity
  - low_severity
  - medium_disruption
  - reboot_required