  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. We construct a covert channel between different computing
systems that utilizes audio modulation/demodulation to exchange data between
the computing systems over the air medium. The underlying network stack is
based on a communication system that was originally designed for robust
underwater communication. We adapt the communication system to implement covert
and stealthy communications by utilizing the ultrasonic frequency range. We
further demonstrate how the scenario of covert acoustical communication over
the air medium can be extended to multi-hop communications and even to 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. 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. Finally, countermeasures against covert acoustical mesh networks
are discussed, including the use of lowpass filtering in computing systems and
a host-based intrusion detection system for analyzing audio input and output in
order to detect any irregularities.
