BlackLotus is a sophisticated piece of malware that can infect a computer's low-level firmware, bypassing the Secure Boot defences built into Windows 10 and Windows 11, and allowing the execution of malicious code before a PC's operating system and security defences have loaded.

In this way, attackers could disable security measures such as BitLocker and Windows Defender, without triggering alarms, and deploy BlackLotus's built-in protection against the bootkit's own removal.

Although Microsoft issued a patch for the flaw in Secure Boot back in January 2022, its exploitation remains possible as the affected, validly-signed binaries have not been added to the UEFI revocation list.

Earlier this year, security researchers explained how BlackLotus was taking advantage of this, "bringing its own copies of legitimate – but vulnerable – binaries to the system in order to exploit the vulnerability."

According to the NSA, there is "significant confusion" about the threat posed by BlackLotus:

“Some organizations use terms like 'unstoppable,' 'unkillable,' and 'unpatchable' to describe the threat. Other organizations believe there is no threat due to patches that Microsoft released in January 2022 and early 2023 for supported versions of Windows. The risk exists somewhere between both extremes."

According to the NSA's advisory, patching Windows 10 and Windows 11 against the vulnerabilities is only "a good first step."

In its mitigation guide, the agency details additional steps for hardening systems.

However, as they involve changes to how UEFI Secure Boot is configured they should be undertaken with caution - as they cannot be reversed once activated, and could leave current Windows boot media unusable if mistakes are made.

"Protecting systems against BlackLotus is not a simple fix," said NSA platform security analyst Zachary Blum.