BadUSB – Turning devices evil. Once reprogrammed, benign devices can turn malicious in many ways, including:

1. 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.
2. The device can also spoof a network card and change the computer’s DNS setting to redirect traffic.
3. 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.