cyberstrikeus Ensure udf kernel module is not available
Profile
Level 2 - Server, Level 2 - Workstation, Assessment: Automated
Description
The udf filesystem type is the universal disk format used to implement ISO/IEC 13346 and ECMA-167 specifications. This is an open vendor filesystem type for data storage on a broad range of media. This filesystem type is necessary to support writing DVDs and newer optical disc formats.
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
Microsoft Azure requires the usage of udf.
udf should not be disabled on systems run on Microsoft Azure.
Audit Procedure
Command Line
Verify the udf kernel module is not available on the system - OR - has been disabled.
Run the following script to determine if the udf kernel module is available on the system:
#!/usr/bin/env bash
{
l_mod_name="udf" 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 udf kernel module is not available on the system and no further audit steps are required.
Note: Some systems may include the udf 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 udf kernel module is not loaded and not loadable by performing the following:
Run the following command to verify the udf kernel module is not loaded:
# lsmod | grep 'udf'
Nothing should be returned
Run the following command to verify the udf kernel module is not loadable:
# modprobe --showconfig | grep -P -- '\b(install|blacklist)\h+udf\b'
Verify the output includes:
blacklist udf
-AND-
install udf /bin/false
-OR-
install udf /bin/true
Expected Result
blacklist udf
install udf /bin/false
Remediation
Command Line
Run the following commands to unload the udf kernel module:
# modprobe -r udf 2>/dev/null
# rmmod udf 2>/dev/null
Perform the following to disable the udf kernel module:
Create a file ending in .conf with install udf /bin/false in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "install udf /bin/false" >> udf.conf
Create a file ending in .conf with blacklist udf in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "blacklist udf" >> udf.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