More Android phones than ever are covertly listening for inaudible sounds in adsEnlarge (credit: Arp et al.)
Almost a year after app developer SilverPush vowed to kill its privacy-threatening software that used inaudible sound embedded into TV commercials to covertly track phone users, the technology is more popular than ever, with more than 200 Android apps that have been downloaded millions of times from the official Google Play market, according to a recently published research paper.
As of January, there were 234 Android apps that were created using SilverPush’s publicly available software developer kit, according to the paper, which was published by researchers from Technische Universitat Braunschweig in Germany.

That represents a dramatic increase in the number of Android apps known to use the creepy audio tracking scheme.
In April 2015, there were only five such apps.

The apps silently listen for ultrasonic sounds that marketers use as high-tech beacons to indicate when a phone user is viewing a TV commercial or other type of targeted audio.

A representative sample of just five of the 234 apps have been downloaded from 2.25 million to 11.1 million times, according to the researchers, citing official Google Play figures. None of them discloses the tracking capabilities in their privacy policies.
“The example of SilverPush highlights how easily this technology can be used to spy on users,” Erwin Quiring, one of the researchers, wrote in an e-mail. “In this way, they can track the TV viewing habits of users precisely even with traditional broadcasting technologies.
In our research paper, we identified three further privacy risks that can occur with this technology, e.g., tracking locations, behavior devices, and even the de-anonymization of Tor users.”
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