canvas fingerprinting
n. A technique for tracking a user online that involves drawing a hidden element on the browser canvas to create a token that uniquely identifies the user’s computer.
canvas fingerprint n.

Example Citations:
First documented in a forthcoming paper by researchers at Princeton University and KU Leuven University in Belgium, this type of tracking, called canvas fingerprinting, works by instructing the visitor’s Web browser to draw a hidden image. Because each computer draws the image slightly differently, the images can be used to assign each user’s device a number that uniquely identifies it.
—Julia Angwin, “Meet the Online Tracking Device That is Virtually Impossible to Block,” ProPublica, July 21, 2014

The mechanism, called “canvas fingerprinting,” uses special scripts — the coded instructions that tell your browser how to render a website — to exploit the browser’s so-called ‘canvas’, a browser functionality that can be used to draw images and render text.
—“Computer privacy: Share button may share your browsing history, too,” ScienceDaily, July 22, 2014

Earliest Citation:
It is worthwhile to note that these methods do preserve the same origin policy — if an image from a different origin has been drawn on this canvas, they will throw a SecurityError exception instead of returning pixel data. Therefore, our <canvas> fingerprints must only contain image resources that are under our control.
—Keaton Mowery and Hovav Shacham, “Pixel Perfect: Fingerprinting Canvas in HTML5“ (PDF), Proceedings of Web 2.0 Security & Privacy 2012, May 24, 2012

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