Security researcher Rob Fuller has discovered a unique attack method that can steal PC credentials from Windows and Mac computers, and possibly Linux (currently untested). Fuller’s attack is effective against locked computers on which the user has already logged in.
The researcher used USB-based Ethernet adapters, for which he modified the firmware code to run special software that sets the plug-and-play USB device as the network gateway, DNS, and WPAD servers on the computer it’s connected to.
You can read more at http://goo.gl/5GnfMt.
Moral of this story: don’t use any USB ethernet cables from unknown sources and don’t allow anyone else to install USB ethernet cables on your equipment.