Boot or Logon Autostart Execution: Kernel Modules and Extensions

Adversaries may modify the kernel to automatically execute programs on system boot. Loadable Kernel Modules (LKMs) are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system. For example, one type of module is the device driver, which allows the kernel to access hardware connected to the system. [1] 

When used maliciously, LKMs can be a type of kernel-mode Rootkit that run with the highest operating system privilege (Ring 0). [2] Common features of LKM based rootkits include: hiding itself, selective hiding of files, processes and network activity, as well as log tampering, providing authenticated backdoors and enabling root access to non-privileged users. [3]

Kernel extensions, also called kext, are used for macOS to load functionality onto a system similar to LKMs for Linux. They are loaded and unloaded through kextload and kextunload commands.

Adversaries can use LKMs and kexts to covertly persist on a system and elevate privileges. Examples have been found in the wild and there are some open source projects. [4] [5] [6] [7][8] [9][10] [11]

ID: T1547.006
Sub-technique of:  T1547
Tactics: Persistence, Privilege Escalation
Platforms: Linux, macOS
Permissions Required: root
Data Sources: Process command-line parameters, Process monitoring
Contributors: Anastasios Pingios; Jeremy Galloway; Red Canary
Version: 1.0
Created: 24 January 2020
Last Modified: 30 June 2020

Procedure Examples

Name Description
Skidmap

Skidmap has the ability to install several loadable kernel modules (LKMs) on infected machines.[11]

Mitigations

Mitigation Description
Antivirus/Antimalware

Common tools for detecting Linux rootkits include: rkhunter [18], chrootkit [19], although rootkits may be designed to evade certain detection tools.

Execution Prevention

Application control and software restriction tools, such as SELinux, KSPP, grsecurity MODHARDEN, and Linux kernel tuning can aid in restricting kernel module loading.[12][13][14][15][16] Since macOS High Sierra 10.13, Secure Kernel Extension Loading (SKEL) can also be used to restrict the loading of kernel modules.[17]

Privileged Account Management

Limit access to the root account and prevent users from loading kernel modules and extensions through proper privilege separation and limiting Privilege Escalation opportunities.

Detection

Loading, unloading, and manipulating modules on Linux systems can be detected by monitoring for the following commands:modprobe, insmod, lsmod, rmmod, or modinfo [20] LKMs are typically loaded into /lib/modules and have had the extension .ko ("kernel object") since version 2.6 of the Linux kernel. [21]

For macOS, monitor for execution of kextload commands and correlate with other unknown or suspicious activity.

Adversaries may run commands on the target system before loading a malicious module in order to ensure that it is properly compiled. [3] Adversaries may also execute commands to identify the exact version of the running Linux kernel and/or download multiple versions of the same .ko (kernel object) files to use the one appropriate for the running system.[11] Many LKMs require Linux headers (specific to the target kernel) in order to compile properly. These are typically obtained through the operating systems package manager and installed like a normal package. On Ubuntu and Debian based systems this can be accomplished by running: apt-get install linux-headers-$(uname -r) On RHEL and CentOS based systems this can be accomplished by running: yum install kernel-devel-$(uname -r)

References