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Scenario: A Windows user with Bitlocker-encrypted OS drive uses Win+L shortcut to lock their computer, but leaves it powered on. At this point, their house is raided (by police, or thieves, or FBI, simply someone with desire to gain access to the data and technical means to mount a proper attempt to do it).

Question: Was there ever a known case, where an attacker with physical access to running computer with Bitlocker turned on was able to bypass the Windows login screen and gain access to Bitlocker-protected data on the computer, while the computer was already running and was only locked, and without powering the device down during the process?

Note: I am aware of DMA and coldboot attacks, or even the method of wiring the TPM to a FPGA board. These attacks usually require the target device to be powered off, at which point any other mounted volumes (for example, through VeraCrypt) would have been lost. My point is that any attacker trying to overcome the Bitlocker is not going to preemptively guess that there's a mounted VeraCrypt volume behind the Bitlocker and will power the device down at some point in an attempt to get through the Bitlocker first. Other questions on this site also pertain to scenarios where sensitive data is stored on the Bitlocker-protected device itself, but my question stems from scenario where data is stored on different disks, using different encryption and Bitlocker-enabled Windows OS is just a gateway.

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A good introduction to the subject is Ivor Kollár (2010): Forensic RAM dump image analyser, and this answer is mainly based on his work. The chapter 2.1.1 describes OS independent methods that do apply to this case of a locked Windows computer:

  • Contrary to what you have understood, Direct Memory Access DMA attacks don't require powering off the computer. If you can dump the memory from the PCI BUS (or IEEE 1394, as the device is technically on the PCI BUS), it's possible to capture the keys even for the mounted VeraCrypt volumes. Cons: The computer may crash during the attempt.

  • Both cold boot and hot boot attacks are similar physical attacks where the goal is to dump as much of the memory as possible. Although the computer is restarted, the attack is based on the hope that the memory doesn't necessarily get emptied. Cons: These might not be practical in real forensic cases, and you also need some luck for these to be successful.

Extracting the cryptographic keys from the memory dump might not be easy and straighforward, but once you have got the contents of the memory using these methods you have unlimited time to investigate them without any fear of losing the keys, if they were there.

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    I would not have thought someone in my country has done a thesis on this very topic. Thanks for this. – masiton May 17 '20 at 21:41
  • After reading through the material, it made me realize that every ordinary running device with cryptographic keys stored in memory is by design vulnerable to a DMA attack, so it matters little whether there's a combination of Bitlocker and VeraCrypt, or not. Once the attacker has enough time and access to copy contents of RAM and has the resources to analyze it, they will be able to extract the keys. Meaningful defense is to unmount any VeraCrypt-protected volume immediately after the work is done, presuming no other threats (keyloggers, sniffers, viruses already present, etc). – masiton May 18 '20 at 13:52
  • The best option is to use a power-on-password with the TPM and turn off (or hibernate, but not sleep) the computer when it's left unattended. – Esa Jokinen May 18 '20 at 13:55
  • I would argue the hibernation is equivalent of dismounting a VeraCrypt volume, but lenghtier. It makes no difference to use preboot authentication with Bitlocker, if the crucial data is stored on VeraCrypt anyway and User has dismounted before sleeping. But in scenario with Bitlocker only, I believe you give valid advice. – masiton May 18 '20 at 13:58

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