Joanna Rutkowska has published a tool that enables an attacker to boot a protected laptop from a USB drive, record the encryption tool s passphrase and then decrypt its contents. The attack is simple enough to be pulled off by a hotel housekeeper and is effective against TrueCrypt. The tool installs a sniffer on the laptop, which will then log the encryption passphrase the next time the user enters it. The passphrase can be stored on the disk and then recovered by the attacker later.
Source: https://threatpost.com/how-defeat-full-disk-encryption-one-minute-101909/72345/