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The researchers note that the chain of attack “is rather complicated,” but it’s not beyond the skills and abilities already seen in advanced attacks conducted by hackers in China and elsewhere. Or by the NSA.

Generally the most common method for infecting air-gapped machines is a USB flash drive or other removable media. Once one air-gapped machine is infected, the malware can spread to other machines on an air-gapped network. Data can be extracted the same way, though this is more of a challenge. The malware stores stolen data on the machine until a flash drive is inserted, at which point data is copied to the drive. When the flash drive is then inserted into another computer that’s connected to the internet, the data gets transmitted back to the attackers’ command-and-control center. This method takes time, however, since it requires the attacker to wait until someone inserts a flash drive into the air-gapped machine and carries it to an internet-connected machine.

AirHopper, however, doesn’t require repeated action like this once the malware is installed. An attacker only needs to get their malicious transmitter code onto the targeted machine and then either install the malicious receiver component on the victim’s mobile phone or use the attacker’s own mobile phone in the vicinity of the computer to receive the data and transmit it to the attacker’s command-and-control server. The malware can be programmed to store siphoned data on the infected machine for later transmission at specified hours or intervals. The researchers also devised methods for hiding the data transmission on the targeted machine to avoid detection, including transmitting data only when the monitor is turned off or in sleep mode and altering the FM receiver on the phone so that there is no audible tone when data is transmitted to it.

Although the distance for transmitting data from an infected computer to a mobile phone is limited—due to the limitations of the receiver in phones—attackers could use a stronger portable receiver, set up in a parking lot for example or installed on a drone flying overhead, to pick up data from greater distances.

There are other limitations, however. The proof-of-concept test allows for data to be transmitted at only 60 bytes a second—about a line of text per second—which limits the speed and volume at which attackers could siphon data. But Mimran notes that over time, a lot of sensitive data can still be extracted this way.