Pre-OS Boot: System Firmware
Adversaries may modify system firmware to persist on systems.The BIOS (Basic Input/Output System) and The Unified Extensible Firmware Interface (UEFI) or Extensible Firmware Interface (EFI) are examples of system firmware that operate as the software interface between the operating system and hardware of a computer.   
System firmware like BIOS and (U)EFI underly the functionality of a computer and may be modified by an adversary to perform or assist in malicious activity. Capabilities exist to overwrite the system firmware, which may give sophisticated adversaries a means to install malicious firmware updates as a means of persistence on a system that may be difficult to detect.
|Hacking Team UEFI Rootkit|
Check the integrity of the existing BIOS or EFI to determine if it is vulnerable to modification. Use Trusted Platform Module technology.  Move system's root of trust to hardware to prevent tampering with the SPI flash memory. Technologies such as Intel Boot Guard can assist with this. 
|Privileged Account Management||
Prevent adversary access to privileged accounts or access necessary to perform this technique.
Patch the BIOS and EFI as necessary.
System firmware manipulation may be detected.  Dump and inspect BIOS images on vulnerable systems and compare against known good images.  Analyze differences to determine if malicious changes have occurred. Log attempts to read/write to BIOS and compare against known patching behavior.
Likewise, EFI modules can be collected and compared against a known-clean list of EFI executable binaries to detect potentially malicious modules. The CHIPSEC framework can be used for analysis to determine if firmware modifications have been performed.   
- Wikipedia. (n.d.). BIOS. Retrieved January 5, 2016.
- Wikipedia. (2017, July 10). Unified Extensible Firmware Interface. Retrieved July 11, 2017.
- UEFI Forum. (n.d.). About UEFI Forum. Retrieved January 5, 2016.
- Lin, P. (2015, July 13). Hacking Team Uses UEFI BIOS Rootkit to Keep RCS 9 Agent in Target Systems. Retrieved December 11, 2015.
- ESET. (2018, September). LOJAX First UEFI rootkit found in the wild, courtesy of the Sednit group. Retrieved July 2, 2019.
- Ge, L. (2011, September 9). BIOS Threat is Showing up Again!. Retrieved November 14, 2014.
- Trusted Computing Group. (2008, April 29). Trusted Platform Module (TPM) Summary. Retrieved June 8, 2016.
- Intel. (2013). Intel Hardware-based Security Technologies for Intelligent Retail Devices. Retrieved May 19, 2020.
- Upham, K. (2014, March). Going Deep into the BIOS with MITRE Firmware Security Research. Retrieved January 5, 2016.
- Butterworth, J. (2013, July 30). Copernicus: Question Your Assumptions about BIOS Security. Retrieved December 11, 2015.
- Beek, C., Samani, R. (2017, March 8). CHIPSEC Support Against Vault 7 Disclosure Scanning. Retrieved March 13, 2017.
- Intel. (2017, March 18). CHIPSEC Platform Security Assessment Framework. Retrieved March 20, 2017.
- Intel Security. (2005, July 16). HackingTeam's UEFI Rootkit Details. Retrieved March 20, 2017.