1.1.1.2 Ensure mounting of squashfs filesystems is disabled - blacklist

Information

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 utilizes 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.

Solution

Run the following script to disable squashfs:

#!/usr/bin/env bash

{
l_mname='squashfs' # set module name
if ! modprobe -n -v '$l_mname' | grep -P -- '^h*install /bin/(true|false)'; then
echo -e ' - setting module: '$l_mname' to be not loadable'
echo -e 'install $l_mname /bin/false' >> /etc/modprobe.d/'$l_mname'.conf
fi
if lsmod | grep '$l_mname' > /dev/null 2>&1; then
echo -e ' - unloading module '$l_mname''
modprobe -r '$l_mname'
fi
if ! grep -Pq -- '^h*blacklisth+$l_mnameb' /etc/modprobe.d/*; then
echo -e ' - deny listing '$l_mname''
echo -e 'blacklist $l_mname' >> /etc/modprobe.d/'$l_mname'.conf
fi
}

See Also

https://workbench.cisecurity.org/files/4068

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6, 800-53|CM-7, CSCv7|9.2

Plugin: Unix

Control ID: 63ee3658512d5acc8f6ffb49e6c54341ab1985d3d9282d7c7c58ced65691973e