Adversaries may abuse Pre-OS Boot mechanisms as a way to establish persistence on a system. During the booting process of a computer, firmware and various startup services are loaded before the operating system. These programs control flow of execution before the operating system takes control.(Citation: Wikipedia Booting)

Adversaries may overwrite data in boot drivers or firmware such as BIOS (Basic Input/Output System) and The Unified Extensible Firmware Interface (UEFI) to persist on systems at a layer below the operating system. This can be particularly difficult to detect as malware at this level will not be detected by host software-based defenses.
Pre-OS Boot
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1542  external_id: T1542     source_name: ITWorld Hard Disk Health Dec 2014  description: Pinola, M. (2014, December 14). 3 tools to check your hard drive's health and make sure it's not already dying on you. Retrieved October 2, 2018.  url: https://www.itworld.com/article/2853992/3-tools-to-check-your-hard-drives-health-and-make-sure-its-not-already-dying-on-you.html     source_name: Wikipedia Booting  description: Wikipedia. (n.d.). Booting. Retrieved November 13, 2019.  url: https://en.wikipedia.org/wiki/Booting     