In 2023, malware tracked as BlackLotus became the first publicly documented in-the-wild UEFI bootkit capable of bypassing Secure Boot on fully updated Windows 11 systems. With the industry-wide chain-of-trust mechanism subverted, the malware was able to run malicious code before Windows was fully loaded and many security defenses were operational.
Microsoft had already patched the vulnerability that BlackLotus exploited in 2022. In 2023 and 2024, Microsoft issued additional updates. The first added a new certificate that no longer trusted the vulnerable boot managers. The second provided the means to revoke trust in the digital certificate used to sign older vulnerable Windows boot managers. The organizations infected by the new bootkit, which is tracked under the name BlackX, had installed all of these patches. So how did they fall victim?
UEFI bootkit refresher
Before discussing the incident further, a brief review of BlackLotus and UEFI malware is in order. Short for Unified Extensible Firmware Interface, UEFI is firmware that runs immediately after a computer is turned on. One of its main jobs is to load system configuration settings, identify and initialize hardware components, and prepare the computer to start its OS.
Another important UEFI function is Secure Boot, a security feature that establishes a chain of trust by verifying the digital signatures of each piece of boot software before allowing it to execute. Its purpose is to prevent attackers who gain control of a device, either physically or through a malware compromise, from running unauthorized boot code in place of legitimate software.
One item that must be digitally signed is the boot manager, a component stored on a small dedicated partition on a drive. The boot manager is responsible for the initial loading of the OS. Both BlackLotus and BlackX exploit “Baton Drop,” a logic flaw in the Windows boot manager that allows attackers to bypass Secure Boot (and, under certain conditions, extract BitLocker encryption keys). The vulnerability, tracked as CVE-2022-21894, came to light in 2022. Microsoft fixed the vulnerability that same year, but it didn’t revoke trust in the certificate used to sign older vulnerable boot managers. BlackLotus exploited this gap and went on to load a malicious kernel-mode driver.
