A little-known web standard that lets site owners tell how much battery life a mobile device has left has been found to enable tracking online, a year after privacy researchers warned that it had the potential to do just that.

The battery status API was introduced in HTML5, the fifth version of the code used to lay out the majority of the web, and had already shipped in Firefox, Opera and Chrome by August 2015. It allows site owners to see the percentage of battery life left in a device, as well as the time it will take to discharge or the time it will take to charge, if connected to a power source.

Intended to allow site owners to serve low-power versions of sites and web apps to users with little battery capacity left, soon after it was introduced, privacy researchers pointed out that it could also be used to spy on users. The combination of battery life as a percentage and battery life in seconds provides offers 14m combinations, providing a pseudo-unique identifier for each device.
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Now, two security researchers from Princeton University have shown that the battery status indicator really is being used in the wild to track users. By running a specially modified browser, Steve Engelhard and Arvind Narayanan found two tracking scripts that used the API to “fingerprint” a specific device, allowing them to continuously identify it across multiple contexts.

Source: Your battery status is being used to track you online | Technology | The Guardian