cyberstrikeus Ensure squashfs kernel module is not available
Profile
Level 2 - Server, Level 2 - Workstation, Assessment: Automated
Description
The squashfs filesystem type is a compressed read-only Linux filesystem embedded in small footprint systems. 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. If this filesystem type is not needed, disable it.
Impact
As Snap packages utilize squashfs as a compressed filesystem, disabling squashfs will cause Snap packages to fail.
Snap application packages of software are self-contained and work across a range of Linux distributions. This is unlike traditional Linux package management approaches, like APT or RPM, which require specifically adapted packages per Linux distribution on an application update and delay therefore application deployment from developers to their software's end-user. Snaps themselves have no dependency on any external store ("App store"), can be obtained from any source and can be therefore used for upstream software deployment.
Note: On operating systems where squashfs is pre-build into the kernel:
- This is considered an acceptable "passing" state
- The kernel should not be re-compiled to remove
squashfs - This audit will return a passing state with "module: "squashfs" doesn't exist in ..."
Audit Procedure
Command Line
Verify the squashfs kernel module is not available on the system - OR - has been disabled.
Run the following script to determine if the squashfs kernel module is available on the system:
#!/usr/bin/env bash
{
l_mod_name="squashfs" l_mod_type="fs"
while IFS= read -r l_mod_path; do
if [ -d "$l_mod_path/${l_mod_name}/-/\/" ] && [ -n "$(ls -A "$l_mod_path/${l_mod_name}/-/\/")" ]; then
printf '%s\n' "$l_mod_name exists in $l_mod_path"
fi
done < <(readlink -f /usr/lib/modules/**/kernel/$l_mod_type || readlink -f /lib/modules/**/kernel/$l_mod_type)
}
If nothing is returned, the squashfs kernel module is not available on the system and no further audit steps are required.
Note: Some systems may include the squashfs filesystem as part of the kernel opposed to being available as a kernel module. In this case, the above audit will not return anything. This is also considered a passing state.
If anything is returned, verify the squashfs kernel module is not loaded and not loadable by performing the following:
Run the following command to verify the squashfs kernel module is not loaded:
# lsmod | grep 'squashfs'
Nothing should be returned
Run the following command to verify the squashfs kernel module is not loadable:
# modprobe --showconfig | grep -P -- '\b(install|blacklist)\h+squashfs\b'
Verify the output includes:
blacklist squashfs
-AND-
install squashfs /bin/false
-OR-
install squashfs /bin/true
Expected Result
blacklist squashfs
install squashfs /bin/false
Remediation
Command Line
Run the following commands to unload the squashfs kernel module:
# modprobe -r squashfs 2>/dev/null
# rmmod squashfs 2>/dev/null
Perform the following to disable the squashfs kernel module:
Create a file ending in .conf with install squashfs /bin/false in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "install squashfs /bin/false" >> squashfs.conf
Create a file ending in .conf with blacklist squashfs in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "blacklist squashfs" >> squashfs.conf
Default Value
Not disabled by default.
References
- NIST SP 800-53 Rev. 5: CM-7
CIS Controls
v8 - 4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software (IG 2, IG 3) v7 - 9.2 Ensure Only Approved Ports, Protocols and Services Are Running (IG 2, IG 3)
MITRE ATT&CK Mappings: T1005, T1005.000 | TA0005 | M1050