FYI: Your browser can pick up ultrasonic signals you can't hear, and that sounds like a privacy nightmare to some

High-frequency audio could be used to stealthily track netizens

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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 apps listening 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.

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Weiler suggested that internet users might be explicitly prompted to enable Web Audio API usage to process sound that can't be heard. His concern is that undetectable audio transmissions could be used for device fingerprinting, for identifying when two different devices are in proximity of each other, and for violating context boundaries that prevent different apps on the same device from talking secretly to one another.

He also asked about masked sounds within the audible spectrum that might be abused for covert communication, though that's a separate technical challenge.

Weiler raised the subject three weeks ago – one element in a larger debate about reducing the fingerprinting surface of the Web Audio API. And last week, the discussion thread was closed by Raymond Toy, a Google software engineer and co-chair of the W3C's Audio Working Group.

Toy argued that if a developer is allowed to use a specific audio sampling rate, no additional permission should be required – few users enjoy dealing with permission prompts, after all. And other web developers participating in the debate expressed concern that limiting available frequency ranges could introduce phase shifting or latency and that there's no sensible lower or upper threshold suitable for everyone.

In an email to The Register, Peter E. Snyder, privacy researcher at Brave software and co-chair of the PING, said he shared Weiler's concerns about the privacy implications of inaudible sound.

"[With regard] to Web Audio and super-audible sounds, we're concerned because audio beyond human perception can be used for a variety of privacy harming purposes," said Snyder. "Companies like SilverPush have commercialized such techniques, and others have documented them being used in the wild.

"Such techniques could also be used to do cross-domain tracking; sites could transmit super-audible sounds that other open pages could listen for, allowing for the kind of cross-site tracking Brave (and other privacy-focused browsers) try to protect users against."

On a related note, the Brave browser recently added a small amount of randomization to various Web Audio APIs to reduce their utility for browser fingerprinting.

Whether or not the Web Audio Working Group decides to revisit the possibility of audio frequency-based permissions, those involved will have their hands full dealing with all other other unaddressed browser privacy worries. ®

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