Boot or Logon Autostart Execution: Kernel Modules and Extensions
Other sub-techniques of Boot or Logon Autostart Execution (11)
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]
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
- Pomerantz, O., Salzman, P.. (2003, April 4). The Linux Kernel Module Programming Guide. Retrieved April 6, 2018.
- Pomerantz, O., Salzman, P. (2003, April 4). Modules vs Programs. Retrieved April 6, 2018.
- Chuvakin, A. (2003, February). An Overview of Rootkits. Retrieved April 6, 2018.
- Case, A. (2012, October 10). Phalanx 2 Revealed: Using Volatility to Analyze an Advanced Linux Rootkit. Retrieved April 9, 2018.
- Kurtz, G. (2012, November 19). HTTP iframe Injecting Linux Rootkit. Retrieved December 21, 2017.
- Augusto, I. (2018, March 8). Reptile - LMK Linux rootkit. Retrieved April 9, 2018.
- 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.
- Wardle, P. (2015, April). Malware Persistence on OS X Yosemite. Retrieved April 6, 2018.
- Wardle, P. (2017, September 8). High Sierra’s ‘Secure Kernel Extension Loading’ is Broken. Retrieved April 6, 2018.
- Mikhail, K. (2014, October 16). The Ventir Trojan: assemble your MacOS spy. Retrieved April 6, 2018.
- Remillano, A., Urbanec, J. (2019, September 19). Skidmap Linux Malware Uses Rootkit Capabilities to Hide Cryptocurrency-Mining Payload. Retrieved June 4, 2020.
- Vander Stoep, J. (2016, April 5). [v3] selinux: restrict kernel module loadinglogin register. Retrieved April 9, 2018.
- Wikibooks. (2018, August 19). Grsecurity/The RBAC System. Retrieved June 4, 2020.
- Kernel.org. (2020, February 6). Kernel Self-Protection. Retrieved June 4, 2020.
- Boelen, M. (2015, October 7). Increase kernel integrity with disabled Linux kernel modules loading. Retrieved June 4, 2020.
- Pingios, A.. (2018, February 7). LKM loading kernel restrictions. Retrieved June 4, 2020.
- Apple. (2018, April 19). Technical Note TN2459: User-Approved Kernel Extension Loading. Retrieved June 30, 2020.
- Rootkit Hunter Project. (2018, February 20). The Rootkit Hunter project. Retrieved April 9, 2018.
- Murilo, N., Steding-Jessen, K. (2017, August 23). Chkrootkit. Retrieved April 9, 2018.
- Henderson, B. (2006, September 24). How To Insert And Remove LKMs. Retrieved April 9, 2018.
- Wikipedia. (2018, March 17). Loadable kernel module. Retrieved April 9, 2018.