Technical folks looking to improve web privacy haven’t been able to decide whether sound beyond the range of human hearing poses enough of a privacy risk to merit restriction.
People can generally hear audio frequencies ranging from 20 Hz and 20,000 Hz, though individual hearing ranges vary. Audio frequencies below and above the threshold of human hearing are known as infrasound and ultrasound, respectively.
A few years ago, digital ad companies began using ultrasonic signals to track people’s interests across devices: if a TV advert, for example, emits a sneaky inaudible signal, a nearby smartphone could pick it up and pass it to an app, which updates the owner’s ad-targeting profile with details of what they were watching and when. Now you know when someone’s into cooking shows on the telly, or is a news junkie, or likes crime documentaries, and so on.
A warning from America’s trade watchdog, the FTC, in 2016 and research published the following year identifying 234 Android appslistening covertly for ultrasound beacons, helped discourage inaudible tracking.
Several of the companies called out for these privacy-invading practices, such as SilverPush, have moved on to other sorts of services. But the ability to craft code that communicates silently with mobile devices through inaudible sound remains a possibility, both for native apps and web apps. Computer security researchers continue to find novel ways to use inaudible audio for data exfiltration. And ultrasound is still used for legitimate operations – Google’s Cast app, for example, relies on an ultrasonic token when pairing with a nearby Chromecast.
Samuel Weiler, a web security engineer with MIT CSAIL and a member of the W3C’s Privacy Interest Group (PING), recently pushed to re-open a discussion about limiting the Web Audio API so that it cannot be used to generate or listen for ultrasonic signals without permission.
Weiler suggested that…
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