cyberstrikeus Ensure cramfs kernel module is not available
Profile
Level 1 - Server, Level 1 - Workstation, Assessment: Automated
Description
The cramfs filesystem type is a compressed read-only Linux filesystem embedded in small footprint systems. A cramfs 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.
Audit Procedure
Command Line
Verify the cramfs kernel module is not available on the system - OR - has been disabled.
Run the following script to determine if the cramfs kernel module is available on the system:
#!/usr/bin/env bash
{
l_mod_name="cramfs" 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 cramfs kernel module is not available on the system and no further audit steps are required.
Note: Some systems may include the cramfs 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 by the above script, verify the cramfs kernel module is not loaded and not loadable by performing the following:
Run the following command to verify the cramfs kernel module is not loaded:
# lsmod | grep 'cramfs'
Nothing should be returned
Run the following command to verify the cramfs kernel module is not loadable:
# modprobe --showconfig | grep -P -- '\b(install|blacklist)\h+cramfs\b'
Verify the output includes:
blacklist cramfs
-AND EITHER-
install cramfs /bin/false
-OR-
install cramfs /bin/true
Expected Result
blacklist cramfs
install cramfs /bin/false
Remediation
Command Line
Run the following to unload and disable the cramfs kernel module.
# modprobe -r cramfs 2>/dev/null
# rmmod cramfs 2>/dev/null
Perform the following to disable the cramfs kernel module:
Create a file ending in .conf with install cramfs /bin/false in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "install cramfs /bin/false" >> cramfs.conf
Create a file ending in .conf with blacklist cramfs in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "blacklist cramfs" >> cramfs.conf
Default Value
Not disabled by default.
References
- NIST SP 800-53: CM-7 a
- NIST SP 800-53A :: CM-7.1 (ii)
- RHEL 8 STIG Group ID: V-230498
- RHEL 9 STIG Group ID: V-257880
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