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The problem isn’t limited to thumb drives. All manner of USB devices from keyboards and mice to smartphones have firmware that can be reprogrammed—in addition to USB memory sticks, Nohl and Lell say they’ve also tested their attack on an Android handset plugged into a PC. And once a BadUSB-infected device is connected to a computer, Nohl and Lell describe a grab bag of evil tricks it can play. It can, for example, replace software being installed with with a corrupted or backdoored version. It can even impersonate a USB keyboard to suddenly start typing commands. “It can do whatever you can do with a keyboard, which is basically everything a computer does,” says Nohl.The malware can silently hijack internet traffic too, changing a computer’s DNS settings to siphon traffic to any servers it pleases. Or if the code is planted on a phone or another device with an internet connection, it can act as a man-in-the-middle, secretly spying on communications as it relays them from the victim’s machine.Most of us learned long ago not to run executable files from sketchy USB sticks. But old-fashioned USB hygiene can’t stop this newer flavor of infection: Even if users are aware of the potential for attacks, ensuring that their USB’s firmware hasn’t been tampered with is nearly impossible. The devices don’t have a restriction known as “code-signing,” a countermeasure that would make sure any new code added to the device has the unforgeable cryptographic signature of its manufacturer. There’s not even any trusted USB firmware to compare the code against.The element of Nohl and Lell’s research that elevates it above the average theoretical threat is the notion that the infection can travel both from computer to USB and vice versa. Any time a USB stick is plugged into a computer, its firmware could be reprogrammed by malware on that PC, with no easy way for the USB device’s owner to detect it. And likewise, any USB device could silently infect a user’s computer. “It goes both ways,” Nohl says. “Nobody can trust anybody.”But BadUSB’s ability to spread undetectably from USB to PC and back raises questions about whether it’s possible to use USB devices securely at all. “We’ve all known if that you give me access to your USB port, I can do bad things to your computer,” says University of Pennsylvania computer science professor Matt Blaze. “What this appears to demonstrate is that it’s also possible to go the other direction, which suggests the threat of compromised USB devices is a very serious practical problem.”Blaze speculates that the USB attack may in fact already be common practice for the NSA. He points to a spying device known as Cottonmouth, revealed earlier this year in the leaks of Edward Snowden. The device, which hid in a USB peripheral plug, was advertised in a collection of NSA internal documents as surreptitiously installing malware on a target’s machine. The exact mechanism for that USB attack wasn’t described. “I wouldn’t be surprised if some of the things [Nohl and Lell] discovered are what we heard about in the NSA catalogue.”
