Adversaries may use bootkits to persist on systems. Bootkits reside at a layer below the operating system and may make it difficult to perform full remediation unless an organization suspects one was used and can act accordingly.

A bootkit is a malware variant that modifies the boot sectors of a hard drive, including the Master Boot Record (MBR) and Volume Boot Record (VBR). (Citation: Mandiant M Trends 2016) The MBR is the section of disk that is first loaded after completing hardware initialization by the BIOS. It is the location of the boot loader. An adversary who has raw access to the boot drive may overwrite this area, diverting execution during startup from the normal boot loader to adversary code. (Citation: Lau 2011)

The MBR passes control of the boot process to the VBR. Similar to the case of MBR, an adversary who has raw access to the boot drive may overwrite the VBR to divert execution during startup to adversary code.
Bootkit
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1542/003  external_id: T1542.003     source_name: Mandiant M Trends 2016  description: Mandiant. (2016, February 25). Mandiant M-Trends 2016. Retrieved March 5, 2019.  url: https://www.fireeye.com/content/dam/fireeye-www/current-threats/pdfs/rpt-mtrends-2016.pdf     source_name: Lau 2011  description: Lau, H. (2011, August 8). Are MBR Infections Back in Fashion? (Infographic). Retrieved November 13, 2014.  url: http://www.symantec.com/connect/blogs/are-mbr-infections-back-fashion     