USB devices are connected to – and in many cases even built into – virtually all computers. The interface standard conquered the world over the past two decades thanks to its versatility: Almost any computer peripheral, from storage and input gadgets to healthcare devices, can connect over the ubiquitous technology. And many more device classes connect over USB to charge their batteries.
This versatility is also USB’s Achilles heel: Since different device classes can plug into the same connectors, one type of device can turn into a more capable or malicious type without the user noticing.
Reprogramming USB peripherals. To turn one device type into another, USB controller chips in peripherals need to be reprogrammed. Very widely spread USB controller chips, including those in thumb drives, have no protection from such reprogramming.
BadUSB – Turning devices evil. 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.
Source and Full Story Here: https://srlabs.de/badusb/