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AliExpress was quietly running audio through some shoppers' browsers that they couldn't hear, potentially giving the online marketplace another way to recognize their devices.
The strange tracking technique came to light after a developer noticed that his multipoint Bluetooth headphones stopped switching properly between his computer and phone whenever he had AliExpress open in Firefox or Chrome.[1] Closing the tab immediately fixed the problem.
His investigation uncovered something much stranger: AliExpress was using the Web Audio API, a legitimate browser feature for processing sound, as part of a broader browser-fingerprinting system.
The code generated and processed audio inside the browser, then reduced its volume to zero. You wouldn't have heard a thing.
Tiny differences in how different devices and browsers handle that audio can provide another signal that helps distinguish one device from another. Combined with information about your graphics hardware, screen, browser, and other characteristics, those signals can contribute to a digital fingerprint.
And muting the AliExpress tab didn't stop the audio processing.
Browser fingerprinting allows websites and third parties to collect characteristics about your device and combine them into a profile that can help distinguish your browser from others.
Those characteristics can include your browser version, operating system, screen resolution, graphics hardware, and other technical details. Fingerprinting can also continue providing identifying signals when traditional tracking methods, such as cookies, aren't available.
Audio fingerprinting adds another piece to that puzzle.
According to the developer's analysis, two obfuscated scripts associated with Alibaba's AWSC anti-abuse infrastructure created separate audio-processing contexts. They generated a known waveform, examined the processed result, reduced the final gain to zero, and connected the audio graph to the computer's audio output.
Different processors, audio hardware, operating systems, browsers, and drivers can introduce tiny variations in how the same audio signal is processed, according to TechSpot.
The scripts examined characteristics associated with canvas rendering, WebGL, display settings, hardware configuration, WebRTC behavior, and user interactions. Combining many relatively weak signals can create a much more detailed fingerprint than any single measurement could provide.
The developer's analysis also found that the collected information was serialized, encrypted, and transmitted. However, the client-side code alone does not reveal exactly how Alibaba uses the information on its servers, how long it retains it, or whether it uses the resulting data to track individual shoppers across Alibaba properties.
Fingerprinting systems can serve legitimate security purposes, but their ability to gather information quietly naturally raises privacy concerns. Particularly when users have no indication that the collection is happening.
If a website is doing something strange with audio, hitting mute might seem like an obvious solution.
Unfortunately, that didn't work here.
The researcher found that muting the AliExpress tab, the entire browser, or Windows failed to stop the behavior affecting his Bluetooth headphones.
The fingerprinting process relied on the way the browser and device processed the audio internally. The audio graph remained connected to the system's output even with its gain reduced to zero.
The technique also didn't require microphone permission. The scripts generated their own audio signal and analyzed how the device processed it, allowing the process to run without recording sounds from your surroundings.
You have a few options if you don't want websites quietly using techniques like this to fingerprint your device.
Use a browser with fingerprinting protections. Brave said its browser “stops” the AliExpress scripts involved in this case. According to TechSpot, Brave has also built protections specifically designed to make audio-fingerprinting results less useful to trackers.
Use a reputable content blocker. The researcher successfully tested narrowly targeted uBlock Origin rules that prevented the two observed AliExpress script families from loading. However, those scripts may change over time, and blocking anti-abuse systems could interfere with features such as logins, checkouts, or fraud checks.
Turn on your browser's fingerprinting protections. Firefox users can use Mozilla's built-in fingerprinting protections, while privacy-focused browsers may offer additional defenses designed to reduce the amount of identifying information websites can collect.
Keep your browser updated. Browser makers continue to develop defenses against fingerprinting and other tracking techniques. Keeping your browser current gives you access to its latest security and privacy protections.
Remember that clearing cookies has limits. Cookies are one way companies can recognize returning users and devices. Fingerprinting can instead draw on characteristics generated by your hardware, software, browser, and behavior.
The AliExpress discovery is a useful reminder that online tracking doesn't always come with a cookie pop-up, a location request, or a website asking for access to your microphone.
Sometimes, your browser can be giving away information without making a sound.
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