LAS VEGAS—During the Cold War, Soviet spies were able to monitor the US Embassy in Moscow by using a radioretroreflector bug—a device powered, like modern RFID tags, by a directed radio signal. But that was too old school for Ang Cui, chief scientist at Red Balloon Security and a recent PhD graduate of Columbia University. He wanted to see if he could do all of that with software.

Building on a long history of research into TEMPEST emanations—the accidental radio signals given off by computing systems’ electrical components—Cui set out to create intentional radio signals that could be used as a carrier to broadcast data to an attacker even in situations where networks were “air-gapped” from the outside world. The result of the work of his research team is Funtenna, a software exploit he demonstrated at Black Hat today that can turn a device with embedded computing power into a radio-based backchannel to broadcast data to an attacker without using Wi-Fi, Bluetooth, or other known (and monitored) wireless communications channels.

Cui has previously demonstrated a number of ways to exploit embedded systems, including printers and voice-over-IP phones. In 2012, he demonstrated an exploit of Cisco phones that turned on the microphone and transformed phones into a remote listening device. Michael Ossmann of Great Scott Gadgets, a hardware hacker who has done some development of exploits based on concepts from the NSA's surveillance "playset," suggested to Cui that he could turn the handset cord of the phone into a “funtenna”—an improvised broadcast antenna generating radio frequency signals programmatically.

With just seven lines of code injected into the embedded computer of an otherwise unmodified laser printer, Cui was able to turn the printer into a radio transmitter by simply leveraging the electrical properties of existing input and output ports on the printer. By rapidly flipping the power state of general purpose input/output (GPIO) pins, Pulse Width Modulation (PWM) outputs, and UART (serial) outputs on a Pantum P2502W laser printer—“the cheapest laser printer we could find,” Cui said—the Funtenna hack was able to create a modulated radio signal as a result of the magnetic fields created by the voltage and resulting electromagnetic waves.

The hack couldn’t generate signals strong enough using the relatively short wires of the GPIO connections on the printer. Despite flipping every GPIO output available, he only got an effective range of transmission of a few meters. Instead, the UART output with a 10-foot cable generated a signal that could be picked up from outside a building—even through reinforced concrete based on Cui's research.

The demonstration, Cui said, shows that embedded devices need their own built-in defenses to truly be secure. And printers are merely a starting point for Cui's work. The same sort of attack could conceivably be launched from any “internet of things” device or other system with onboard computing power—including network routers and firewalls.

“A network [intrusion detection system] is no substitute for host-based defense,” he said. “You could monitor every known spectrum, but it would be very expensive and may not work. The best way is to have host-based defense baked into every embedded device.”

Listing image by Red Balloon Security