Hunting Bootkits in the EFI System Partition

Overview

The EFI System Partition (ESP) is a small FAT32-formatted partition that the platform firmware reads at power-on to locate and execute the operating system's boot loader. Because it executes before the operating system, kernel, and any EDR agent, the ESP is one of the most coveted persistence locations for advanced adversaries. UEFI bootkits that live on the ESP survive OS reinstallation, disk reformatting of the OS partition, and most endpoint defenses.

This skill follows the detection research published by Eclypsium ("Enhanced Threat Detection: Bootloaders, Bootkits, and Secure Boot", and the Bootkitty and Glupteba analyses) and aligns with the ESP-hunting methodology Rapid7 documented for Velociraptor (Windows.Forensics.UEFI, Windows.Detection.Yara.UEFI). The threat context is concrete and current:

• ESPecter (ESET, 2021) — a UEFI bootkit that persists on the ESP as a patched Windows Boot Manager (bootmgfw.efi) plus malicious kernel-mode drivers. It marked the move of UEFI threats from SPI flash to the ESP, where they are far easier to deploy.
• BlackLotus (2023) — the first in-the-wild UEFI bootkit able to bypass Secure Boot on fully patched Windows 11 by exploiting CVE-2022-21894 ("baton drop") and staging a vulnerable signed bootmgfw.efi on the ESP.
• Bootkitty (2024) — the first UEFI bootkit targeting Linux, dropped onto the ESP.
• Glupteba — commodity malware whose UEFI variant replaces software on the EFI partition.

The core detection insight from Eclypsium and Rapid7 is that the bootloader normally changes only during a vendor or OS update; an out-of-band change to ESP binaries, an unsigned or untrusted-signed boot loader, or any file in the ESP root that is not under the EFI/ directory is a high-fidelity indicator of compromise. This skill builds that baseline and hunts deviations from it.

When to Use