Alibaba's cross-border e-commerce platform, AliExpress, has been discovered running an inaudible audio signal in the background of its web pages, using the device's processing of this sound to supplement its identification of visitor device characteristics. The discovery began with a pair of Bluetooth earphones.

A developer noticed that his earphones, which support simultaneous connection to both a computer and a mobile phone, would behave oddly. Normally, when the computer has no audio playing, the phone can play music without interruption. However, whenever the computer's browser opened the AliExpress homepage, the music on the phone would stop abruptly. Closing that tab immediately restored the earphones to normal function.

There was no video, no music, and no play button on the page. Even after muting the tab, the browser, and the system volume, the problem persisted. Upon investigation, the developer found that the web page creates two WebAudio audio contexts that remain in an active state. The sound passes through a node with zero volume, making it inaudible to users, but it still connects to the system's audio output, causing the browser to continue processing the audio. For multi-point connected earphones, this makes the computer appear as though it is still playing content.

The silent audio can also serve as a device fingerprinting tool. When the same audio segment is processed by different devices, subtle differences arise in the CPU, sound card, browser, and driver components. By measuring these differences and combining them with information such as screen size, fonts, and browser environment, the website can form a relatively stable device fingerprint. This entire process does not rely on cookies, nor does it require activating the microphone for recording.

Many people are learning for the first time that web pages can identify devices through the audio system. Some have speculated about whether phones are being eavesdropped on, but this investigation found no evidence of recording. What it measures is the device's processing result of a silent signal. The earphones' refusal to switch back to the phone unexpectedly exposed the "soundless audio" running in the background.