Pre-OS Boot
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. 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.
Open detection, hunting, mitigation, and evidence workspace
Detection logic
Perform integrity checking on pre-OS boot mechanisms that can be manipulated for malicious purposes. Take snapshots of boot records and firmware and compare against known good images. Log changes to boot records, BIOS, and EFI, which can be performed by API calls, and compare against known good behavior and patching. Disk check, forensic utilities, and data from device drivers (i.e. processes and API calls) may reveal anomalies that warrant deeper investigation.