Researchers have created proof-of-concept (PoC) malware that can stealthily exfiltrate data from air-gapped computers using power lines.

The malware, dubbed PowerHammer, is the work of researchers at the Ben-Gurion University of the Negev in Israel. The university has previously published research on jumping air gaps via magnetic fields, infrared cameras, router LEDs, scanners, HDD activity LEDs, USB devices, the noise emitted by hard drives and fans, and heat emissions.

PowerHammer exfiltrates data from a compromised machine by regulating its power consumption, which can be controlled through the workload of the device’s CPU. Sensitive pieces of information, such as passwords and encryption keys, can be stolen one bit at a time by modulating changes in the current flow.

Researchers have devised two versions of the PowerHammer attack: line level power-hammering and phase level power-hammering.

In the line level variant, the attacker intercepts the bits of data exfiltrated by the malware by tapping the compromised computer’s power cable. In the phase level attack, the attacker collects the data from the main electrical service panel. The data can be harvested using a non-invasive tap that measures the emissions on power cables, and converting them to a binary form via demodulation and decoding.

A computer’s CPU is a significant power consumer and its workload has a direct impact on power consumption and implicitly the flow of current in the device’s power cable. By overloading the CPU with calculations and stopping and starting the workload, it’s possible to generate a signal over the power lines at a specified frequency.

In the case of PowerHammer, the attacker establishes two different frequencies – one representing a “0” bit and another frequency representing a “1” bit.

During their experiments, researchers saw transfer rates of up to 1000 bits/sec for the line level power-hammering attack and 10 bits/sec for the phase level attack. The best transfer rates were achieved on a PC, followed by a server (which had lower bit rates and more errors), and IoT devices (bit rates of up to 20 bits/sec and error rates of up to 18%).

While these can be significant transfer rates for exfiltrating small pieces of information such as passwords – obtaining one character from a string requires 8 bits to be transferred – reliable exfiltration requires more than just sending the raw data. Researchers created 44-bit data frames that, in addition to the actual data being exfiltrated, include a preamble that signals the start of the transmission and 8 bits of CRC code at the end of the frame for error detection.

As for countermeasures, researchers say PowerHammer attacks can be prevented by monitoring power lines for the presence of covert communication channels, by using power line filters to limit the leakage of conduction and radiation noise, and by installing software-level jammers that execute random workloads on the system in order to cause interference in the data transmission process.

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