Windows BitLocker bug exposes AES-XTS encryption

A medium-severity bug in the Windows BitLocker full-disk encryption tool has exposed the BitLocker encryption system to a novel randomization attack that has targeted the AES-XTS encryption mode.

The new vulnerability — CVE-2025-21210 — underscores the increased sophistication of attacks on full-disk encryption systems. Upon exploitation, it can let attackers manipulate ciphertext blocks, causing sensitive data to be written to disk in plaintext.

Jason Soroko, senior fellow at Sectigo, explained that BitLocker uses AES-XTS encryption to ensure that even if someone physically accesses the hard drive, they can’t easily read the data books without the key.

Rather than stealing or directly reading the books, the attacker subtly alters certain pages (the ciphertext blocks) in multiple books, Soroko explained while using an analogy to describe randomization attacks. When a page gets tampered with, the rest of the book remains intact and unreadable, he continued.

However, by carefully targeting and repeatedly modifying these pages in just the right spots, the attacker can cause the library’s system to occasionally misplace or expose sensitive information from the books, Soroko said. Over time, this can lead to parts of the data being written down in plain language, effectively leaking confidential information without direct decryption.

“The real danger is that this method doesn’t require breaking the encryption directly,” said Soroko. “Instead, it manipulates how the encrypted data is handled, allowing attackers to bypass security measures and access sensitive information. Teams should ensure that their encryption software is up-to-date with the latest security patches, restrict physical access to devices, and monitor systems for any unusual activity that might indicate tampering.”