Two newly discovered bugs in the Microsoft Windows XP Universal Serial Bus [USB] driver mean that a simple USB storage device can be turned into what is essentially a hardware-based Trojan. SPI Dynamics security engineers David Dewey and Darrin Barrall have revealed a hack where full data compromise can be achieved in less than 10 seconds of physical access.

Citing the example of a retail point-of-sale terminal with a USB port on the monitor, a malicious attacker can discretely plug in the USB device, wait 10 seconds while a monitoring program downloads and then leave the scene. Subsequently, after a time period of a week or so has elapsed, the USB device is plugged back in and the recorded transaction and credit card information is pulled off the terminal for "two, 10-second attacks that no one ever saw."
This type of attack can only occur with Windows AutoRun functionality, and only works on non-removable devices; however it is possible to make a USB device look non-removable via in-system programming. So be careful what you plug into that port!