Detections
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1.1.1.1 Ensure cramfs kernel module is not available
Information
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.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 unload and disable the cramfs module:- IF - the cramfs kernel module is available in ANY installed kernel:
- Create a file ending inconf with install cramfs /bin/false in the /etc/modprobe.d/ directory
- Create a file ending inconf with blacklist cramfs in the /etc/modprobe.d/ directory
- Run modprobe -r cramfs 2>/dev/null; rmmod cramfs 2>/dev/null to remove cramfs from the kernel
- IF - the cramfs kernel module is not available on the system, or pre-compiled into the kernel, no remediation is necessary
#!/usr/bin/env bash
{
unset a_output2; l_output3="" l_dl="" # unset arrays and clear variables
l_mod_name="cramfs" # set module name
l_mod_type="fs" # set module type
l_mod_path="$(readlink -f /lib/modules/**/kernel/$l_mod_type | sort -u)"
f_module_fix()
{
l_dl="y" # Set to ignore duplicate checks
a_showconfig=() # Create array with modprobe output
while IFS= read -r l_showconfig; do
a_showconfig+=("$l_showconfig")
done < <(modprobe --showconfig | grep -P -- 'b(install|blacklist)h+'"${l_mod_name//-/_}"'b')
if lsmod | grep "$l_mod_name" &> /dev/null; then # Check if the module is currently loaded
a_output2+=(" - unloading kernel module: \"$l_mod_name\"")
modprobe -r "$l_mod_name" 2>/dev/null; rmmod "$l_mod_name" 2>/dev/null
fi
if ! grep -Pq -- 'binstallh+'"${l_mod_name//-/_}"'h+/bin/(true|false)b' <<< "${a_showconfig[*]}"; then
a_output2+=(" - setting kernel module: \"$l_mod_name\" to \"/bin/false\"")
printf '%s
' "install $l_mod_name /bin/false" >> /etc/modprobe.d/"$l_mod_name".conf
fi
if ! grep -Pq -- 'bblacklisth+'"${l_mod_name//-/_}"'b' <<< "${a_showconfig[*]}"; then
a_output2+=(" - denylisting kernel module: \"$l_mod_name\"")
printf '%s
' "blacklist $l_mod_name" >> /etc/modprobe.d/"$l_mod_name".conf
fi
}
for l_mod_base_directory in $l_mod_path; do # Check if the module exists on the system
if [ -d "$l_mod_base_directory/${l_mod_name/-//}" ] && [ -n "$(ls -A $l_mod_base_directory/${l_mod_name/-//})" ]; then
l_output3="$l_output3
- \"$l_mod_base_directory\""
[[ "$l_mod_name" =~ overlay ]] && l_mod_name="${l_mod_name::-2}"
[ "$l_dl" != "y" ] && f_module_fix
else
echo -e " - kernel module: \"$l_mod_name\" doesn't exist in \"$l_mod_base_directory\""
fi
done
[ -n "$l_output3" ] && echo -e "
-- INFO --
- module: \"$l_mod_name\" exists in:$l_output3"
[ "${#a_output2[@]}" -gt 0 ] && printf '%s
' "${a_output2[@]}"
echo -e "
- remediation of kernel module: \"$l_mod_name\" complete
"
}
See Also
Item Details
Category: CONFIGURATION MANAGEMENT
References: 800-53|CM-6, 800-53|CM-7, CSCv7|9.2
Plugin: Unix
Control ID: be92911691d2a8c560acd32294c10f383b752e5e7397b3af5cb8b314d54a9d03