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.(Citation: Linux Kernel Programming) 

When used maliciously, LKMs can be a type of kernel-mode [Rootkit](https://attack.mitre.org/techniques/T1014) that run with the highest operating system privilege (Ring 0).(Citation: Linux Kernel Module Programming Guide) 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.(Citation: iDefense Rootkit Overview)

Kernel extensions, also called kext, are used in macOS to load functionality onto a system similar to LKMs for Linux. Since the kernel is responsible for enforcing security and the kernel extensions run as apart of the kernel, kexts are not governed by macOS security policies. Kexts are loaded and unloaded through <code>kextload</code> and <code>kextunload</code> commands. Kexts need to be signed with a developer ID that is granted privileges by Apple allowing it to sign Kernel extensions. Developers without these privileges may still sign kexts but they will not load unless SIP is disabled. If SIP is enabled, the kext signature is verified before being added to the AuxKC.(Citation: System and kernel extensions in macOS)

Since macOS Catalina 10.15, kernel extensions have been deprecated in favor of System Extensions. However, kexts are still allowed as "Legacy System Extensions" since there is no System Extension for Kernel Programming Interfaces.(Citation: Apple Kernel Extension Deprecation)

Adversaries can use LKMs and kexts to conduct [Persistence](https://attack.mitre.org/tactics/TA0003) and/or [Privilege Escalation](https://attack.mitre.org/tactics/TA0004) on a system. Examples have been found in the wild, and there are some relevant open source projects as well.(Citation: Volatility Phalanx2)(Citation: CrowdStrike Linux Rootkit)(Citation: GitHub Reptile)(Citation: GitHub Diamorphine)(Citation: RSAC 2015 San Francisco Patrick Wardle)(Citation: Synack Secure Kernel Extension Broken)(Citation: Securelist Ventir)(Citation: Trend Micro Skidmap)

Platforms: macOS
Linux
Tactics:
phase_name: persistence
phase_name: privilege-escalation

Detection:
Loading, unloading, and manipulating modules on Linux systems can be detected by monitoring for the following commands: <code>modprobe</code>, <code>insmod</code>, <code>lsmod</code>, <code>rmmod</code>, or <code>modinfo</code> (Citation: Linux Loadable Kernel Module Insert and Remove LKMs) LKMs are typically loaded into <code>/lib/modules</code> and have had the extension .ko ("kernel object") since version 2.6 of the Linux kernel. (Citation: Wikipedia Loadable Kernel Module)

Adversaries may run commands on the target system before loading a malicious module in order to ensure that it is properly compiled. (Citation: iDefense Rootkit Overview) 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.(Citation: Trend Micro Skidmap) 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: <code>apt-get install linux-headers-$(uname -r)</code> On RHEL and CentOS based systems this can be accomplished by running: <code>yum install kernel-devel-$(uname -r)</code>

On macOS, monitor for execution of <code>kextload</code> commands and user installed kernel extensions performing abnormal and/or potentially malicious activity (such as creating network connections). Monitor for new rows added in the <code>kext_policy</code> table. KextPolicy stores a list of user approved (non Apple) kernel extensions and a partial history of loaded kernel modules in a SQLite database, <code>/var/db/SystemPolicyConfiguration/KextPolicy</code>.(Citation: User Approved Kernel Extension Pike’s)(Citation: Purves Kextpocalypse 2)(Citation: Apple Developer Configuration Profile)


References:
source_name: mitre-attack
description: Apple. (2019, May 3). Configuration Profile Reference. Retrieved September 23, 2021.
source_name: Apple Developer Configuration Profile
description: Apple. (n.d.). Deprecated Kernel Extensions and System Extension Alternatives. Retrieved November 4, 2020.
source_name: Apple Kernel Extension Deprecation
description: Apple. (n.d.). System and kernel extensions in macOS. Retrieved March 31, 2022.
source_name: System and kernel extensions in macOS
description: Augusto, I. (2018, March 8). Reptile - LMK Linux rootkit. Retrieved April 9, 2018.
source_name: GitHub Reptile
description: Case, A. (2012, October 10). Phalanx 2 Revealed: Using Volatility to Analyze an Advanced Linux Rootkit. Retrieved April 9, 2018.
source_name: Volatility Phalanx2
description: Chuvakin, A. (2003, February). An Overview of Rootkits. Retrieved April 6, 2018.
source_name: iDefense Rootkit Overview
description: Henderson, B. (2006, September 24). How To Insert And Remove LKMs. Retrieved April 9, 2018.
source_name: Linux Loadable Kernel Module Insert and Remove LKMs
description: Kurtz, G. (2012, November 19). HTTP iframe Injecting Linux Rootkit. Retrieved December 21, 2017.
source_name: CrowdStrike Linux Rootkit
description: Mello, V. (2018, March 8). Diamorphine - LMK rootkit for Linux Kernels 2.6.x/3.x/4.x (x86 and x86_64). Retrieved April 9, 2018.
source_name: GitHub Diamorphine
description: Mikhail, K. (2014, October 16). The Ventir Trojan: assemble your MacOS spy. Retrieved April 6, 2018.
source_name: Securelist Ventir
description: Pikeralpha. (2017, August 29). User Approved Kernel Extension Loading…. Retrieved September 23, 2021.
source_name: User Approved Kernel Extension Pike’s
description: Pomerantz, O., Salzman, P. (2003, April 4). Modules vs Programs. Retrieved April 6, 2018.
source_name: Linux Kernel Module Programming Guide
description: Pomerantz, O., Salzman, P.. (2003, April 4). The Linux Kernel Module Programming Guide. Retrieved April 6, 2018.
source_name: Linux Kernel Programming
description: Remillano, A., Urbanec, J. (2019, September 19). Skidmap Linux Malware Uses Rootkit Capabilities to Hide Cryptocurrency-Mining Payload. Retrieved June 4, 2020.
source_name: Trend Micro Skidmap
description: Richard Purves. (2017, November 9). MDM and the Kextpocalypse . Retrieved September 23, 2021.
source_name: Purves Kextpocalypse 2
description: Wardle, P. (2015, April). Malware Persistence on OS X Yosemite. Retrieved April 6, 2018.
source_name: RSAC 2015 San Francisco Patrick Wardle
description: Wardle, P. (2017, September 8). High Sierra’s ‘Secure Kernel Extension Loading’ is Broken. Retrieved April 6, 2018.
source_name: Synack Secure Kernel Extension Broken
description: Wikipedia. (2018, March 17). Loadable kernel module. Retrieved April 9, 2018.
source_name: Wikipedia Loadable Kernel Module