Hackers and spies have a slew of ways to pull data from your computer, but not if it’s disconnected from the Internet. Think again. Scientists have found a way to secretly network computers using sound — no Internet or Bluetooth connection required.

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Michael Hanspach and Michael Goetz, computer scientists at the Fraunhofer Institute in Germany, used a piece of software that takes control of the sound card and emits sound to other computers at frequencies humans can’t hear. The software gets onto the computer the way other forms of malware are often distributed, via an infected USB stick, an email with an attachment or a hyperlink to a nefarious website.

If two or more computers become infected, they can communicate with each other covertly in a way that’s similar to how two computers might communicate using a Wi-Fi network. But since this covert network is virtually silent to a human ear and doesn’t require an Internet or Bluetooth connection, the communication could go undetected. The sound waves broadcast data at about 20 bits per second, which isn’t much, but it’s enough to log keystrokes or send encrypted information between the computers.

While it sounds scary, there are some caveats. The range is limited. Hanspach and Goetz managed to transmit data about 60 feet at the most. So anyone wanting to build this kind of covert network would need to put the software on a lot of closely-spaced computers.

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Also, like any other hacking method, there are countermeasures. An acoustic network can be blocked by adjusting the filters on a computer’s microphone. Simply blocking any frequencies out of the range of human hearing would mean that any communications from an infected computer would have to use audible ones, which would make it pretty obvious something was up.

As Mark Hagerott, a cybersecurity professor at the U.S. Naval Academy in Annapolis, Md., told Inside Science, it’s all part of the arms race between the attackers and defenders of computer systems. While the sound method Hanspach and Goetz designed will only work on a few laptop models, that could be improved, and further research will show which sound frequencies are better and how to increase the range.

Their research appears on wrote in the Journal of Communications.

Credit: Wikimedia Commons/ Ministry of Defence (UK)