Massive, undetectable security flaw found in USB. The problem, according to SR Labs, is that these USB controllers can have their firmware reprogrammed so that they announce themselves as a different class. For example, you could reprogram a mass storage device so that it masquerades as a network controller, so that all of your network communications (websites, passwords) get redirected to the device.

http://www.extremetech.com/computing/187279-undetectable-indefensible-security-flaw-found-in-usb-its-time-to-get-your-ps2-keyboard-out-of-the-cupboard