"Evil Maid" attack against disk encryption

Physical security is important. The "Evil Maid" attack serves as a reminder that briefly allowing a laptop out of your control, even with an encrypted hard disk, means that all security bets are off—the machine should be considered potentially compromised. Obviously different users have different levels of paranoia about their data security, but the Evil Maid attack shows just how simple it can be for others to access your data.

There is nothing particularly new in the proof-of-concept (PoC) attack against TrueCrypt disk encryption software, but the simplicity of the approach should give one pause. Joanna Rutkowska described the attack back in January, but the need for physical computer security goes back much further than that. But, folks are less wary of physical attacks against laptops today because of whole-disk encryption. Rutkowska's PoC, along with last year's report on "cold boot" attacks, should make it clear that encryption—at least without some kind of Trusted Platform Module (TPM) support—is not a complete solution

The basic idea behind Evil Maid is that someone gets access to a laptop for a fairly short period of time (a few minutes), and, in that time, boots it from a USB key. One obvious vector is a hotel maid (or someone acting as one), who enters someone's room while they are out to dinner, which is what gives the attack its name. The USB key contains a payload that hooks the TrueCrypt password prompting code and stores the last password entered. The payload gets added to the Master Boot Record (MBR) of the laptop so that it becomes active on the next boot.

While it has not been implemented in the PoC, there is no reason that the malware couldn't send the password off via the network; currently it just reports it back the next time the Evil Maid USB key is booted. That would require the attacker to access the laptop twice—with its user typing in the encryption key in between—but a multi-day hotel stay would give ample opportunity for that to occur.

As Bruce Schneier points out, this attack is in no way limited to TrueCrypt, as other solutions suffer from the same vulnerabilities. Both Schneier and Rutkowska look at some potential workarounds, but, in the final analysis, physical access allows an attacker too many ways around these security measures. Even Trusted Computing, with appropriate TPM hardware, can succumb to certain kinds of attacks.

Microsoft's BitLocker drive encryption uses the TPM, which provides reasonable assurance that the right code is being booted, but even that can fall prey to Evil Maid-style attacks, as Rutkowska describes:

Rutkowska also describes a "Poor Man's Solution" which calculates hashes of various unencrypted portions of the disk (especially the MBR). The Disk Hasher is a bootable Linux-based USB key that calculates and stores the hashes on the USB key, as well as verifying the correct hashes prior to booting. As she points out, it only protects against disk-based attacks—BIOS reflashing would subvert Disk Hasher.

Requiring a password in the BIOS before booting is another possible workaround, but one that may not provide as much security as it at first seems. BIOS reflashing is one possible attack, but an easier—though more time-consuming than the "standard" Evil Maid attack—method would be to remove the disk, attach it to another laptop and install the necessary code. It also adds complexity to the attack, but the 5-15 minutes needed to swap out a laptop hard disk is not all that difficult to come by in the hotel scenario.

This PoC, along with other attacks against encrypted disks, is very useful to remind users that hard disk encryption is no panacea. You still must consider which kinds of threats you are trying to protect against. Disk encryption is great for preventing accidental disclosure of private information when someone steals a laptop, but is much less useful for an attack that is focused on accessing the data on a particular laptop. Much like internet security, fairly straightforward protection techniques are fine to thwart the random attacker but are probably insufficient for one who is focused on subverting your defenses in particular.