This is an interesting security exploit without a clear or obvious fix:

Today Kamkar released the schematics and code for a proof-of-concept device he calls PoisonTap: a tiny USB dongle that, whether plugged into a locked or unlocked PC, installs a set of web-based backdoors that in many cases allow an attacker to gain access to the victim’s online accounts, corporate intranet sites, or even their router. Instead of exploiting any glaring security flaw in a single piece of software, PoisonTap pulls off its attack through a series of more subtle design issues that are present in virtually every operating system and web browser, making the attack that much harder to protect against.

“In a lot of corporate offices, it’s pretty easy: You walk around, find a computer, plug in PoisonTap for a minute, and then unplug it,” Kamkar says. The computer may be locked, he says, but PoisonTap “is still able to take over network traffic and plant the backdoor.”

Having physical access to a PC generally results in increased risk. So it should not be much of a surprise this is possible from an access perspective, but only from an operating system or browser vulnerability context.