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Android users might be secretly tracked by inaudible sounds hidden in ads

More and more Android apps are trying to track their users without their knowledge, according to a new report from researchers at Technische Universität Braunschweig in Germany.

Companies have started hiding “beacons”, ultrasonic audio signals inaudible to humans, in their adverts so they can keep track of the devices and learn more about their owners, the Independent reports. Electronic devices that have a microphone are able to register these sounds, which allows advertisers to discover their location and discover what kind of ads the users watch on TV, as well as what other devices they own. The technique can even de-anonymise Tor users.

“Throughout our empirical study, we confirm that audio beacons can be embedded in sound, such that mobile devices spot them with high accuracy while humans do not perceive the ultrasonic signals consciously,” reads the report.


The researchers noted that only six apps were known to be using ultrasound tracking technology in April 2015. The number grew to 39 by December 2015, and has increased since then to 234.

While the study hasn’t named specific programs, they mentioned that several have millions of downloads and belong to reputable companies, such as McDonald’s and Krispy Kreme.

“They embed these beacons in the ultrasonic frequency range between 18 and 20 kHz of audio content and detect them with regular mobile applications using the device’s microphone,” the report adds.

However, consumers need to have the apps open in order for advertisers to track their data, so the privacy threat might not be as big as people thought. Researchers expect the problem to grow “in the near future”.

In order to avoid these apps, users should think twice before downloading something that asks for permission to access your camera or microphone.

Daisy Wilder