A researcher has demonstrated that threat actors could exfiltrate data from an air-gapped device over an acoustic channel even if the targeted machine does not have any speakers, by abusing the power supply.
Some isolated systems may have their audio hardware disabled in an effort to prevent stealthy data exfiltration. However, researcher Mordechai Guri from the Cyber-Security Research Center at the Ben-Gurion University of the Negev in Israel has shown that a piece of malware can cause a device’s power supply unit to generate sounds that can be picked up by a nearby receiver.
The malware, which requires no special permissions, can modulate information from the air-gapped device over the acoustic signals and send them to a smartphone that “listens.” This method can be used to exfiltrate passwords, encryption keys, and files from PCs, servers and even IoT devices that have no audio hardware.
Tests conducted by Guri have shown that the attack method, which he has dubbed POWER-SUPPLaY, can be used to steal data from an air-gapped system over a distance of up to 5 meters (16 feet) with a maximum transfer rate of 50 bits per second — the transfer rate decreases as the distance increases.
The attack involves starting and stopping the CPU workload, which influences the switching frequency of the power supply, which in turn impacts the transformers and capacitors in the power supply. These transformers and capacitors generate acoustic signals (i.e. noise).
Guri showed that a piece of malware able to control a device’s CPU workload can accurately cause the power supply to generate both audible and inaudible sounds. The researcher demonstrated that a power supply can play the song “Happy Birthday” using this method.
Of course, a piece of malware whose goal is to silently exfiltrate data would not play any sound that would attract attention. Instead, it would play audible or inaudible sounds on two or more different frequencies, each frequency representing a 0 bit, a 1 bit or a sequence of bits (e.g. 00, 01, 10, 11) that will be captured by a receiver.
The receiving smartphone can be owned by the attacker or it can be a compromised device belonging to someone who works in the targeted organization.
This is not the first time a researcher from the Ben-Gurion University of the Negev has demonstrated a method for exfiltrating data from air-gapped devices. Over the past years, they showed techniques for silently stealing data using fan vibrations, heat emissions, hard drive LEDs, infrared cameras, magnetic fields, power lines, router LEDs, scanners, screen brightness, USB devices, and noise from hard drives and fans.