Researchers break into BitLocker

The security test lab of Fraunhofer SIT has published a technique for getting around Microsoft's BitLocker disk-encryption technology, even when BitLocker is used in connection with a hardware-based Trusted Platform Module.

The attack is intended to counter the widely held belief that a Trusted Platform Module (TPM) device is a foolproof way of protecting sensitive data, Fraunhofer SIT researchers said on Thursday.

"Our attack demonstration does not imply a bug in BitLocker, nor does it render Trusted Computing useless," said Fraunhofer SIT researchers Jan Steffan and Jan Trukenmüller in a statement. "BitLocker still works as well as other disk-encryption products, it only fails to fulfil an unrealistic yet common expectation. "

BitLocker Drive Encryption, found in Vista, Windows 7 and Server 2008 versions of Microsoft Windows, is designed to prevent a thief from viewing protected files by tampering with a lost or stolen PC. If there is a TPM on the computer, this can be used in the encryption and decryption process for extra protection. 

Microsoft told ZDNet UK it was aware of the attack, but could not immediately comment.

The Fraunhofer attack is based on an 'Evil Maid' scenario, in which the attacker — for example, a maid in a hotel — is granted physical access to the machine in question.

Previous Evil Maid attacks, such as the 'Stoned Boot' attack demonstrated by Austrian security specialist Peter Kleissner in July, or the 'Cold Boot' attack demonstrated by Princeton researchers last year, have demonstrated techniques for getting around full-disk encryption protection, but those attacks did not involve the use of a TPM.

The Fraunhofer SIT researchers said they intended to demonstrate that adding a TPM may make an Evil Maid attack more difficult, but that it does not mean a system is completely protected.

"Many people seem to believe that Trusted Computing would automatically protect the system from all software-based attacks against the boot process, and in particular that using BitLocker with a TPM would achieve such protection," stated Steffan and Trukenmüller.

"[But] a variety of hardware-based attacks against BitLocker... remain possible in the Evil Maid scenario. We demonstrate how an attack based solely on tampering with the boot loader may still succeed and help the attacker to gain access to confidential data."

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To carry out the attack, the malicious user first boots the target system from a separate device, such as a USB key. The attacker then replaces the boot code of BitLocker with code designed to record the user-provided key — such as a password — in an unencrypted portion of the hard disk.

The modified boot code then restores the original state of the boot loader and initates a reboot. The attack requires a second visit of the Evil Maid to recover the stored key, which can then be used to gain access to the encrypted data.

Fraunhofer SIT noted that the TPM is far from useless, as it makes the attack more noticeable than it would be otherwise, and  itrequires the attacker to access the machine twice.

"As an application of the Trusted Computing platform, BitLocker uses only a subset of the functions available, and it does so in a particular way. Our attack applies only to the combination of platform, application, attack scenario, and attack objective discussed here," wrote Steffan and Trukenmüller.

Fraunhofer SIT has published a research paper on the attack on its website.

Microsoft told ZDNet UK it was aware of the attack, but could not immediately comment.