Once reprogrammed, benign devices can turn malicious in many ways, including:

A device can emulate a keyboard and issue commands on behalf of the logged-in user, for example to exfiltrate files or install malware. Such malware, in turn, can infect the controller chips of other USB devices connected to the computer.

The device can also spoof a network card and change the computer’s DNS setting to redirect traffic.

A modified thumb drive or external hard disk can – when it detects that the computer is starting up – boot a small virus, which infects the computer’s operating system prior to boot.

via Turning USB peripherals into BadUSB | Security Research Labs.

Looks like Karsten Nohl has done it again!