Computer scientists have proposed a malware prototype that uses inaudible audio signals to communicate, a capability that allows the malware to covertly transmit keystrokes and other sensitive data even when infected machines have no network connection.

The proof-of-concept software—or malicious trojans that adopt the same high-frequency communication methods—could prove especially adept in penetrating highly sensitive environments that routinely place an "air gap" between computers and the outside world. Using nothing more than the built-in microphones and speakers of standard computers, the researchers were able to transmit passwords and other small amounts of data from distances of almost 65 feet. The software can transfer data at much greater distances by employing an acoustical mesh network made up of attacker-controlled devices that repeat the audio signals.

The researchers, from Germany's Fraunhofer Institute for Communication, Information Processing, and Ergonomics, recently disclosed their findings in a paper published in the Journal of Communications. It came a few weeks after a security researcher said his computers were infected with a mysterious piece of malware that used high-frequency transmissions to jump air gaps. The new research neither confirms nor disproves Dragos Ruiu's claims of the so-called badBIOS infections, but it does show that high-frequency networking is easily within the grasp of today's malware.

"In our article, we describe how the complete concept of air gaps can be considered obsolete as commonly available laptops can communicate over their internal speakers and microphones and even form a covert acoustical mesh network," one of the authors, Michael Hanspach, wrote in an e-mail. "Over this covert network, information can travel over multiple hops of infected nodes, connecting completely isolated computing systems and networks (e.g. the internet) to each other. We also propose some countermeasures against participation in a covert network."

The researchers developed several ways to use inaudible sounds to transmit data between two Lenovo T400 laptops using only their built-in microphones and speakers. The most effective technique relied on software originally developed to acoustically transmit data under water. Created by the Research Department for Underwater Acoustics and Geophysics in Germany, the so-called adaptive communication system (ACS) modem was able to transmit data between laptops as much as 19.7 meters (64.6 feet) apart. By chaining additional devices that pick up the signal and repeat it to other nearby devices, the mesh network can overcome much greater distances.

The ACS modem provided better reliability than other techniques that were also able to use only the laptops' speakers and microphones to communicate. Still, it came with one significant drawback—a transmission rate of about 20 bits per second, a tiny fraction of standard network connections. The paltry bandwidth forecloses the ability of transmitting video or any other kinds of data with large file sizes. The researchers said attackers could overcome that shortcoming by equipping the trojan with functions that transmit only certain types of data, such as login credentials captured from a keylogger or a memory dumper.

"This small bandwidth might actually be enough to transfer critical information (such as keystrokes)," Hanspach wrote. "You don't even have to think about all keystrokes. If you have a keylogger that is able to recognize authentication materials, it may only occasionally forward these detected passwords over the network, leading to a very stealthy state of the network. And you could forward any small-sized information such as private encryption keys or maybe malicious commands to an infected piece of construction."

Remember Flame?

The hurdles of implementing covert acoustical networking are high enough that few malware developers are likely to add it to their offerings anytime soon. Still, the requirements are modest when measured against the capabilities of Stuxnet, Flame, and other state-sponsored malware discovered in the past 18 months. And that means that engineers in military organizations, nuclear power plants, and other truly high-security environments should no longer assume that computers isolated from an Ethernet or Wi-Fi connection are off limits.

The research paper suggests several countermeasures that potential targets can adopt. One approach is simply switching off audio input and output devices, although few hardware designs available today make this most obvious countermeasure easy. A second approach is to employ audio filtering that blocks high-frequency ranges used to covertly transmit data. Devices running Linux can do this by using the advanced Linux Sound Architecture in combination with the Linux Audio Developer's Simple Plugin API. Similar approaches are probably available for Windows and Mac OS X computers as well. The researchers also proposed the use of an audio intrusion detection guard, a device that would "forward audio input and output signals to their destination and simultaneously store them inside the guard's internal state, where they are subject to further analyses."

Update

On Wednesday Hanspach issued the following statement:

Fraunhofer FKIE is actively involved in information security research. Our mission is to strengthen security by the means of early detection and prevention of potential threats. The research on acoustical mesh networks in air was aimed at demonstrating the upcoming threat of covert communication technologies. Fraunhofer FKIE does not develop any malware or viruses and the presented proof-of-concept does not spread to other computing systems, but constitutes only a covert communication channel between hypothetical instantiations of a malware. The ultimate goal of the presented research project is to raise awareness for these kinds of attacks, and to deliver appropriate countermeasures to our customers.

Story updated to add "prototype" to the first sentence and headline and to change "developed" to "proposed," in the first sentence. The changes are intended to make clear the researchers have not created a piece of working malware.