The Evil Maid Attack is an attack type against whole system disk encryption in a form of a small bootable USB stick image that allows to perform the attack in an easy “plug-and-play” way. The whole infection process takes about 1 minute, and it’s well suited to be used by hotel maids.
The Invisible Things blog goes into great detail on how most whole disk encryption is vulnerable in a relatively simple way. The scenario we consider is when somebody left an encrypted laptop e.g. in a hotel room. Let’s assume the laptop uses full disk encryption like e.g. this provided by TrueCrypt or PGP Whole Disk Encryption. Many people believe, including some well known security experts, that it is advisable to fully power down your laptop when you use full disk encryption in order to prevent attacks via FireWire/PCMCIA or ”Coldboot” attacks. So, let’s assume we have a reasonably paranoid user, that uses a full disk encryption on his or her laptop, and also powers it down every time they leave it alone in a hotel room, or somewhere else.
Now, this is where our Evil Maid stick comes into play. All the attacker needs to do is to sneak into the user’s hotel room and boot the laptop from the Evil Maid USB Stick. After some 1-2 minutes, the target laptop’s gets infected with Evil Maid Sniffer that will record the disk encryption passphrase when the user enters it next time. As any smart user might have guessed already, this part is ideally suited to be performed by hotel maids, or people pretending to be them.
So, after our victim gets back to the hotel room and powers up his or her laptop, the passphrase will be recorded and e.g. stored somewhere on the disk, or maybe transmitted over the network (not implemented in current version).
Now we can safely steal/confiscate the user’s laptop, as we know how to decrypt it. End of story.