A critical vulnerability in Windows BitLocker, identified as CVE-2025-21210, has exposed the encryption mechanism to a novel randomization attack targeting the AES-XTS encryption mode. 

This vulnerability allows attackers with physical access to manipulate ciphertext blocks, causing sensitive data to be written to disk in plaintext. The flaw underscores the evolving sophistication of attacks against full-disk encryption systems.

BitLocker, a widely used full-disk encryption tool in Microsoft Windows, relies on AES-XTS for encrypting storage devices. 

Unlike its predecessor, AES-CBC, which is vulnerable to bit-flipping attacks that allow precise modifications of decrypted plaintext, AES-XTS randomizes plaintext when ciphertext is altered.

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This makes targeted manipulations theoretically impractical. However, CVE-2025-21210 demonstrates that even AES-XTS is not immune to exploitation under specific conditions.

Maxim Suhanov, a computer forensics expert, explains that the vulnerability exploits a design flaw in how BitLocker handles crash dump configurations.

By corrupting a single registry key (HKLM\System\ControlSet001\Control\CrashControl), attackers can disable the dumpfve.sys crash dump filter driver. 

This forces the Windows kernel to write unencrypted hibernation images directly to disk. These images often contain sensitive data from RAM, such as passwords, encryption keys, and personal information.

The Attack Phases

The attack involves two key stages:

Identifying Target Locations: The attacker must determine the precise disk offsets corresponding to critical registry keys or data structures. This is achieved by observing changes in ciphertext across multiple states of the encrypted disk. 

Randomizing Ciphertext Blocks: Once the target location is identified, the attacker corrupts specific ciphertext blocks. In AES-XTS mode, this randomizes the corresponding plaintext block without affecting others. 

This vulnerability poses significant risks in scenarios where physical access to devices is possible. For example:

Corporate Espionage: An attacker could exploit this flaw on stolen laptops configured with TPM-only BitLocker protection.

Data Recovery Abuse: Devices sent for repair or recycling could be targeted if proper security measures are not enforced.

Although exploitation requires high technical expertise and physical access, the potential impact is severe due to the exposure of sensitive data stored in RAM.

Fixes Available

Microsoft addressed this vulnerability by releasing an updated version of the fvevol.sys driver. 

The patch introduces a validation mechanism that ensures dumpfve.sys remains listed in the DumpFilters registry value. If it is missing or corrupted, Windows will crash immediately during boot-up, preventing unencrypted data from being written to disk.

Users are strongly advised to apply Microsoft’s security patch immediately and implement essential safeguards. 

Organizations must remain vigilant and adopt comprehensive security practices to mitigate risks associated with physical access and cryptographic weaknesses.

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