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Security researchers Michael Hanspach and Michael Goetz from the Fraunhofer Institute for Communication, Information Processing and Ergonomics in Germany, have devised an attack method using ultrasonic sound waves to exploit software for built-in speakers on a system to record keystrokes.

“Covert channels can be used to circumvent system and network policies by establishing communications that have not been considered in the design of the computing system,” the researchers explained. “We construct[ed] a covert channel between different computing systems that utilizes audio modulation/demodulation to exchange data between the computing systems over the air medium.”

The researchers said that the underlying network stack is based on a communication system that was actually designed for underwater communications, and they adapted the system to deliver “covert and stealthy communications” in the ultrasonic frequency range that can be extended to “multi-hop communications” and wireless mesh networks.

“A covert acoustical mesh network can be conceived as a meshed botnet or malnet that is accessible via inaudible audio transmissions. Different applications of covert acoustical mesh networks are presented, including the use for remote keylogging over multiple hops,” they reported.

“It is shown that the concept of a covert acoustical mesh network renders many conventional security concepts useless, as acoustical communications are usually not considered.”

The researchers also discuss possible countermeasures to defeat the threat from covert acoustical mesh networks, including using of lowpass filtering and host-based intrusion detection for monitoring audio input and output to detect any irregularities.

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