USB stands for Universal Serial Bus. Almost every device (Keyboard, storage device, speakers, etc) can be attached over USB these days. Therefore your computer

USB stands for Universal Serial Bus. Almost every device (Keyboard, storage device, speakers, etc) can be attached over USB these days. Therefore your computer can't know what kind of device can attach over USB; it's left to the device itself to tell the computer what it is. The big security problem that they found is that the computer has no way to verify that the usb device is actually the type of device that it proclaims to be. This opens up a massive security hole (as demonstrated at blackhat). A usb device can first tell the computer that it's a mass-storage device, and then later change itself to a keyboard and then start 'typing' commands as a user would. The computer can't see if it's a real keyboard or a fake keyboard, and that's the problem. This is not a vulnerability in your OS but rather a gross oversight in the USB specification. This vulnerability is shown to be cross platform(linux,windows) and cross hardware(2 different usb chipsets). It's dubbed 'unpatchable' because to patch this we need a need new (safe) USB specification and you'd need to buy a new pc with those new usb ports.