Variability in different math libraries, which are used when audio is produced through browsers, were once high enough to provide a massively large number of uniquely different signatures when combined with different CPUs and other system differences. After the audio soundprinting technique became well known, Firefox implemented a fix, starting in version 118 released in 2023. Beginning then, the browser used its own unique math libraries, rather than relying on ones that shipped with the OS it ran on. Tom Ritter, a Firefox developer who has also volunteered for the Tor Project, said the move to constant libraries reduced the entropy enough to stop the technique from working.

The technique is ineffective in Chrome because it, too, ships with its own libraries, a Google spokesperson said. Safari users are likely safe for the same reason, although Apple didn’t immediately confirm that.

An artifact from years past

AliExpress’ use of an obsolete fingerprinting method raises the question: Why is it bothering to use it? The likely answer has to do with the more than a dozen other fingerprinting methods the online retailer is using. Callaghan said the other techniques included:

• canvas rendering and toDataURL()
• WebGL renderer information, extensions, and shader precision
• audio oscillator and analyzer output
• screen and viewport dimensions
• device pixel ratio
• hardware concurrency and device memory
• installed browser plugins
• supported audio and video formats
• WebRTC behavior
• browser performance timing
• mouse, touch, focus, and scroll events
• device motion and orientation
• properties commonly associated with browser automation

It’s likely the web audio trick is left over from years earlier and no one noticed it until now.

It’s comforting to know that browser makers have taken precautions. At the same time, it’s not yet clear how effective the rest of the metrics AliExpress is using are. Less reassuring still: it’s almost a certainty that thousands of sites are employing similar tracking. Browser developers are in a dynamic race with site publishers, who are constantly looking for new ways to break the latest protections.