BitUnlocker Downgrade Attack Bypasses Windows 11 Disk Encryption in Minutes

A proof-of-concept (PoC) exploit that demonstrates how attackers can bypass Windows 11 BitLocker disk encryption in under 5 minutes.

Dubbed the “BitUnlocker” attack, this physical downgrade technique exploits a known vulnerability, CVE-2025-48804.

Initially documented by the Microsoft STORM team in July 2025, the flaw exposes a critical weakness in how Secure Boot interacts with legacy certificates.

Despite Microsoft issuing a patch, the fix remains incomplete for many systems. The primary issue stems from the Microsoft Windows PCA 2011 certificate.

While updated machines received a patched boot manager, the older PCA 2011 certificate was never revoked en masse due to the severe operational disruptions it would cause across global enterprise networks.

Consequently, this older certificate remains perfectly valid within the Secure Boot database of almost all active machines, allowing an old, vulnerable boot manager to load without triggering security alerts.

To execute the BitUnlocker attack, an adversary requires physical access to a target device running a TPM-only BitLocker configuration. The attacker can launch the exploit using a simple USB flash drive or via a network-based PXE boot.

Security researcher Cassius Garat has released an exploit that relies on the preparation of a modified Boot Configuration Data (BCD) file. This file redirects Windows Recovery Environment (WinRE) input to a modified System Deployment Image (SDI).

During the boot process, the system loads the older PCA 2011 signed boot manager. The boot manager checks the first legitimate Windows Imaging Format (WIM) file but simultaneously boots from a second, attacker-controlled WIM hidden inside the SDI.

This malicious WIM contains a compromised WinRE image that runs a terminal command. Because the legacy certificate is still trusted, the Trusted Platform Module (TPM) releases the BitLocker keys as expected, since the platform configuration registers (PCRs 7 and 11) remain valid. Within minutes, a terminal opens, presenting the decrypted operating system volume.

To properly divert the boot sequence, attackers modify the local BCD file using the following commands in the recovery prompt:

bcdedit /export BCD_modded
bcdedit /store BCD_modded /set {default} path \WINDOWS\system32\winload_DOESNOTEXIST.efi

bcdedit /store BCD_modded /enum all
:: Locate the "Windows Recovery" GUID, then execute:
bcdedit /store BCD_modded /set {GUID} ramdisksdidevice boot
bcdedit /store BCD_modded /set {GUID} ramdisksdipath \sdi\boot_patched.sdi

Vulnerability Overview

• Vulnerability-CVE-2025-48804 (BitUnlocker)
• Target System-Windows 11 BitLocker (TPM-only configurations)
• Attack Vector- Physical access using USB or PXE downgrade
• Root Cause-Unrevoked PCA 2011 Certificate in Secure Boot

Because standard Windows updates do not fully eliminate this threat, system administrators must apply specific mitigations to protect encrypted endpoints from downgrade attacks:

• Enable TPM plus PIN Authentication: Configuring a pre-boot PIN prevents the TPM from automatically unsealing the encryption keys. This stops the exploit entirely since it requires explicit user interaction before the boot path can proceed.
• Migrate to Windows UEFI CA 2023: Organizations should aggressively update their boot managers to utilize the new CA 2023 certificate. Once deployed, administrators must explicitly revoke the PCA 2011 certificate using the DBX database. Detailed steps are available in Microsoft KB5025885.
• Modify PCR Policies: Moving away from default settings by setting PCR to 0, 2, or 4 will allow the system to detect unauthorized changes to the boot path, effectively neutralizing the modified SDI file approach.

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