In 2013, a document leaked by former National Security Agency contractor Edward Snowden illustrated how a specially modified USB device allowed spies to surreptitiously siphon data out of targeted computers, even when they were physically severed from the Internet or other networks.

Now, researchers have developed software that goes a step further by turning unmodified USB devices into covert transmitters that can funnel large amounts of information out of similarly "air-gapped" PCs.

USBee, by contrast, turns USB devices already inside the targeted facility into a transmitter with no hardware modification required at all.

"We introduce a software-only method for short-range data exfiltration using electromagnetic emissions from a USB dongle," researchers from Israel's Ben-Gurion University wrote in a research paper published Monday.

"Unlike other methods, our method doesn't require any radio frequency transmitting hardware since it uses the USB's internal data bus."

The software works on just about any storage device that's compliant with the USB 2.0 specification.

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