Disable Mounting of squashfs
Ensures the squashfs kernel module is not loaded and is blacklisted so it cannot be auto-loaded on demand.
Checked against the resolved running state (e.g. sshd -T, sysctl, systemctl show), catches drop-ins and Includes a file read would miss. Caveat: runtime ≠ persistence; a value correct now may not survive a reboot.
Pavois asserts the effective configuration, the live, resolved state, not a file. File-based scanners (OVAL/SCAP, Lynis) miss Includes, drop-ins and runtime defaults; this check sees what is actually applied.
A mapping is a cross-reference to where each standard places this requirement, anchored and cross-validated, not a claim of equivalence. A passing check is evidence toward these references, how to read it.
Why this rule matters
Removing support for unneeded filesystem types reduces the local attack surface of the system. The squashfs driver parses untrusted, attacker-supplied images; any bug in it becomes a local privilege-escalation or denial-of-service vector. Unless the host actually mounts SquashFS images (e.g. Snap packages, which rely on it), the module should be neither loaded nor loadable.
What Pavois checks
Pavois queries the effective module state, not config files: it checks that squashfs is absent from the running kernel and that modprobe is configured to refuse loading it (install squashfs /bin/true plus a blacklist entry). Reading /etc/modprobe.d/*.conf alone would miss a module already loaded at runtime or one pulled in by an unexpected drop-in; querying the live kernel catches both.
describe kernel_module('squashfs') do
it { should_not be_loaded }
it { should be_disabled }
endHow to verify it is applied
lsmod | grep squashfsshould return nothing (module not loaded).modprobe -n -v squashfsshould printinstall /bin/true(loading is neutralized).grep -r squashfs /etc/modprobe.d/should show ablacklist squashfsand aninstall squashfs /bin/trueline.
Inspect & investigate
dmesg | grep -i squashfsshows kernel messages if the module is ever loaded.journalctl -k | grep squashfsfor boot-time module activity.lsmodandmodprobe -n -v squashfsreflect the live state at any time.
Remediation
Pavois's harden plan uses the kernel_module resource to blacklist squashfs: it writes a modprobe drop-in that blacklists the module and redirects its load to /bin/true, then unloads it if currently active. Because the module may already be in use, a reboot is required to fully clear it. Apply with pavois harden apply.
Pavois applies this with its own harden engine, the plan below, not a shell script:
| action | blacklist |
|---|---|
| name | squashfs |
| reboot_required | true |
| resource | kernel_module |
pavois harden plan localwhere the target is local, a user@host SSH alias, or a container , Docs
Impact & precautions
Low risk on servers. Caution: Snap packages depend on squashfs, blacklisting it on a host that uses Snap (common on Ubuntu desktops/some images) will break Snap mounts. Verify with mount | grep squashfs and snap list before applying; if Snap is in use, exclude this rule for that host.
Standards mapping
| Standard | Reference | Type | Version | Confidence |
|---|---|---|---|---|
| CIS | 1.1.1.6, 1.1.1.7 | direct | per OS, see the benchmark table | high |
| NIST | 3.4.6, CM-6(a), CM-7(a), CM-7(b) | supporting | 800-53 Rev 5 · 800-171 Rev 2 (pinned) | medium |
Each reference is a cross-reference anchored in the upstream benchmark and cross-validated against the SCAP Security Guide and ansible-lockdown, not a claim of equivalence. Direct = a prescriptive, line-level requirement; supporting = an abstract control family (NIST) the check provides evidence toward. How to read a mapping.