A few hours after dark one evening earlier this month, a small quadcopter drone lifted off from the parking lot of Ben-Gurion University in Beersheba, Israel. It soon trained its built-in camera on its target, a desktop computer's tiny blinking light inside a third-floor office nearby. The pinpoint flickers, emitting from the LED hard drive indicator that lights up intermittently on practically every modern Windows machine, would hardly arouse the suspicions of anyone working in the office after hours. But in fact, that LED was silently winking out an optical stream of the computer’s secrets to the camera floating outside.

That data-stealing drone, shown in the video below, works as a Mr. Robot-style demonstration of a very real espionage technique. A group of researchers at Ben-Gurion's cybersecurity lab has devised a method to defeat the security protection known as an “air gap,” the safeguard of separating highly sensitive computer systems from the internet to quarantine them from hackers. If an attacker can plant malware on one of those systems—say, by paying an insider to infect it via USB or SD card—this approach offers a new way to rapidly pull secrets out of that isolated machine. Every blink of its hard drive LED indicator can spill sensitive information to any spy with a line of sight to the target computer, whether from a drone outside the window or a telescopic lens from the next roof over.

"If an attacker has a foothold in your air-gapped system, the malware still can send the data out to the attacker," says Ben-Gurion researcher Mordechai Guri, who has spent years focusing on finding techniques for ferreting data out of isolated computer systems. "We found that the small hard drive indicator LED can be controlled at up to 6,000 blinks per second. We can transmit data in a very fast way at a very long distance."

Gap Attack

An air gap, in computer security, is sometimes seen as an impenetrable defense. Hackers can't compromise a computer that's not connected to the internet or other internet-connected machines, the logic goes. But malware like Stuxnet and the Agent.btz worm that infected American military systems a decade ago have proven that air gaps can't entirely keep motivated hackers out of ultra-secret systems—even isolated systems need code updates and new data, opening them to attackers with physical access. And once an air-gapped system is infected, researchers have demonstrated a grab bag of methods for extracting information from them despite their lack of an internet connection, from electromagnetic emanations to acoustic and heat signaling techniques—many developed by the same Ben-Gurion researchers who generated the new LED-spying trick.

But exploiting the computer's hard drive indicator LED has the potential to be a stealthier, higher-bandwidth, and longer-distance form of air-gap-hopping communications. By transmitting data from a computer's hard drive LED with a kind of morse-code-like patterns of on and off signals, the researchers found they could move data as fast as 4,000 bits a second, or close to a megabyte every half hour. That may not sound like much, but it's fast enough to steal an encryption key in seconds. And the recipient could record those optical messages to decode them later; the malware could even replay its blinks on a loop, Guri says, to ensure that no part of the transmission goes unseen.

The technique also isn't as limited in range as other clever systems that transmit electromagnetic signals or ultrasonic noises from speakers or a computer's fans. And compared to other optical techniques that use the computer's screen or keyboard light to secretly transmit information, the hard-drive LED indicator—which blinks anytime a program accesses the hard drive—routinely flashes even when a computer is asleep. Any malware that merely gains the ability of a normal user, rather than deeper administrative privileges, can manipulate it. The team used a Linux computer for their testing, but the effects should be the same on a Windows device.

"The LED is always blinking as it's doing searching and indexing, so no one suspects, even in the night," says Guri. "It’s very covert, actually."

Slow and Steady

The researchers found that when their program read less than 4 kilobytes from the computer's storage at a time, they could cause the hard drive's LED indicator to blink for less than a fifth of a millisecond. They then tried using those rapid fire blinks to send messages to a variety of cameras and light sensors from an "infected" computer using a binary system of data encoding known as "on-off-keying," or OOK. They found that a typical smartphone camera can at most receive around 60 bits per second due to its lower frame rate, while a GoPro camera captured as much as 120 bits per second. A Siemens photodiode sensor was far better suited to their high-frequency light sensing needs, though, and allowed them to hit their 4,000 bits per second maximum transmission rate.

The malware could also make the hard drive LED blink so briefly, in fact, that it would be undetectable to human eyes, yet still registered by the light sensor. That means an attacker could even send invisible light signals to a faraway spy, albeit at a slower rate to avoid its covert blinks blurring into a visible signal. "It's possible for the attacker to do such fast blinking that a human never sees it," says Guri.

The good news, however, for anyone security-sensitive enough to worry about the researchers' attack—and anyone who air gaps their computers may be just that sensitive—is that the Ben Gurion researchers point to clear countermeasures to block their hard drive LED exfiltration method. They suggest keeping air-gapped machines in secure rooms away from windows, or placing film over a building's glass designed to mask light flashes. They also note that protective software on a target machine could randomly access the hard drive to create noise and jam any attempt to send a message from the computer's LED.

But the simplest countermeasure by far is simply to cover the computer's LED itself. Once, a piece of tape over a laptop's webcam was a sign of paranoia. Soon, a piece of tape obscuring a computer's hard drive LED may be the real hallmark of someone who imagines a spy drone at every window.