Detections
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1.1.1.7 Ensure udf kernel module is not available

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Information

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.

Removing support for unneeded filesystem types reduces the local attack surface of the system. If this filesystem type is not needed, disable it.

Solution

Run the following script to disable the udf module:

-IF- the module is available in the running kernel:

 - Create a file ending in .conf with install udf /bin/false in the /etc/modprobe.d/ directory
 - Create a file ending in .conf with blacklist udf in the /etc/modprobe.d/ directory
 - Unload udf from the kernel

-IF- available in ANY installed kernel:

 - Create a file ending in .conf with blacklist udf in the /etc/modprobe.d/ directory

-IF- the kernel module is not available on the system or pre-compiled into the kernel:

 - No remediation is necessary

#!/usr/bin/env bash

{
 l_mname="udf" # set module name
 l_mtype="fs" # set module type
 l_mpath="/lib/modules/**/kernel/$l_mtype"
 l_mpname="$(tr '-' '_' <<< "$l_mname")"
 l_mndir="$(tr '-' '/' <<< "$l_mname")"

 module_loadable_fix()
 {
 # If the module is currently loadable, add "install {MODULE_NAME} /bin/false" to a file in "/etc/modprobe.d"
 l_loadable="$(modprobe -n -v "$l_mname")"
 [ "$(wc -l <<< "$l_loadable")" -gt "1" ] && l_loadable="$(grep -P -- "(^\h*install|\b$l_mname)\b" <<< "$l_loadable")"
 if ! grep -Pq -- '^\h*install \/bin\/(true|false)' <<< "$l_loadable"; then
 echo -e "\n - setting module: \"$l_mname\" to be not loadable"
 echo -e "install $l_mname /bin/false" >> /etc/modprobe.d/"$l_mpname".conf
 fi
 }
 module_loaded_fix()
 {
 # If the module is currently loaded, unload the module
 if lsmod | grep "$l_mname" > /dev/null 2>&1; then
 echo -e "\n - unloading module \"$l_mname\""
 modprobe -r "$l_mname"
 fi
 }
 module_deny_fix()
 {
 # If the module isn't deny listed, denylist the module
 if ! modprobe --showconfig | grep -Pq -- "^\h*blacklist\h+$l_mpname\b"; then
 echo -e "\n - deny listing \"$l_mname\""
 echo -e "blacklist $l_mname" >> /etc/modprobe.d/"$l_mpname".conf
 fi
 }
 # Check if the module exists on the system
 for l_mdir in $l_mpath; do
 if [ -d "$l_mdir/$l_mndir" ] && [ -n "$(ls -A $l_mdir/$l_mndir)" ]; then
 echo -e "\n - module: \"$l_mname\" exists in \"$l_mdir\"\n - checking if disabled..."
 module_deny_fix
 if [ "$l_mdir" = "/lib/modules/$(uname -r)/kernel/$l_mtype" ]; then
 module_loadable_fix
 module_loaded_fix
 fi
 else
 echo -e "\n - module: \"$l_mname\" doesn't exist in \"$l_mdir\"\n"
 fi
 done
 echo -e "\n - remediation of module: \"$l_mname\" complete\n"
}

Impact:

Microsoft Azure requires the usage of udf.

udf should not be disabled on systems run on Microsoft Azure.

See Also

https://workbench.cisecurity.org/benchmarks/17074

Item Details

References: CSCv7|9.2

Plugin: Unix

Control ID: 1ba086800bcc8220b39f8833798ece069d838ebbd3b52c12a0d8cd6c4f72b6c4