Microsoft shares mitigation for YellowKey Windows zero-day

Microsoft has shared mitigations for YellowKey, a recently disclosed Windows BitLocker zero-day vulnerability that grants access to protected drives.

The security flaw was disclosed last week by an anonymous security researcher known as 'Nightmare Eclipse,' who described it as a backdoor and published a proof-of-concept (PoC) exploit.

Nightmare Eclipse said that exploiting this zero-day involves placing specially crafted 'FsTx' files on a USB drive or EFI partition, rebooting into WinRE, and then triggering a shell with unrestricted access to the BitLocker-protected storage volume by holding down the CTRL key.

Last month, they also disclosed the BlueHammer (CVE-2026-33825) and RedSun (no identifier) local privilege escalation (LPE) zero-day flaws, both of which are now being exploited in attacks.

The researcher also leaked GreenPlasma, a zero-day privilege-escalation security issue that attackers can abuse to obtain a SYSTEM shell, and UnDefend, another zero-day that attackers with standard user permissions can exploit to block Microsoft Defender definition updates.

While the exact circumstances that triggered this spree of exploit leaks are still unclear, Nightmare Eclipse previously said that these disclosures are in protest of how Microsoft's Security Response Center (MSRC) handled the disclosure process for other security flaws they reported in the past.

Microsoft shares YellowKey mitigations

On Tuesday, Microsoft said it is now tracking the YellowKey flaw under CVE-2026-45585 and shared mitigation measures to defend against potential attacks exploiting it in the wild.

"Microsoft is aware of a security feature bypass vulnerability in Windows publicly referred to as "YellowKey". The proof of concept for this vulnerability has been made public violating coordinated vulnerability best practices," Microsoft said in a Tuesday advisory.

"We are issuing this CVE to provide mitigation guidance that can be implemented to protect against this vulnerability until the security update is made available."

To mitigate YellowKey attacks, Microsoft recommended removing the autofstx.exe entry from the Session Manager's BootExecute REG_MULTI_SZ value, then reestablishing BitLocker trust for WinRE by following the procedure detailed under "Mitigations" in the CVE-2026-33825 advisory.

"Specifically, you prevent the FsTx Auto Recovery Utility, autofstx.exe, from automatically starting when the WinRE image launches," Will Dormann, principal vulnerability analyst at Tharros, explained. "With this change, the Transactional NTFS replaying that deletes winpeshl.ini no longer happens."

Microsoft also advised customers to configure BitLocker on already encrypted devices from "TPM-only" mode to "TPM+PIN" mode via PowerShell, the command line, or the control panel, which will require a pre-boot PIN to decrypt the drive at startup and should block YellowKey attacks.

On devices that are not yet encrypted, admins can enable the "Require additional authentication at startup" option via Microsoft Intune or Group Policies, while ensuring that "Configure TPM startup PIN" is set to "Require startup PIN with TPM."

The Validation Gap: Automated Pentesting Answers One Question. You Need Six.

Automated pentesting tools deliver real value, but they were built to answer one question: can an attacker move through the network? They were not built to test whether your controls block threats, your detection rules fire, or your cloud configs hold.

This guide covers the 6 surfaces you actually need to validate.

Download Now