Security professionals love to talk about the "air gap" as the ultimate in safety for a computer: When it's not attached to network cables or a wireless network, it's presumed to be safe. Presumed, that is, until now. This week, researchers from the Ben-Gurion University of the Negev announced that they have come up with a way to exfiltrate data from air-gapped computers via malware that can control the computer's power consumption.

By adding workload to CPU cores that aren't doing anything else, the malware will change how much power (how many watts) the computer is using. Done carefully, the result is, essentially, an FM transmission over the power line. When a probe is placed near the power cable, the modulation can be detected and decoded — and information will have left the system.

The researchers call the malware that controls the power consumption PowerHammer; so far, it's a research proof-of-concept that hasn't been seen in the wild. That's good, because while ways to thwart a PowerHammer-like attack exist, none are perfect.

PowerHammer isn't the first time control or information signals have been sent over power lines. Electric motors are frequently controlled via pulse-width modulation (PWM) sent over the power lines, building control systems have used power-line carriers, and some electrical utilities have experimented with broadband internet access over power lines. This is, however, a reminder that capabilities can be used by individuals and groups with many different agendas.

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