Firefox to Block Canvas-based Browser Fingerprinting
1.11.2017 securityweek Privacy
Firefox will soon provide users with increased privacy by blocking browser fingerprinting performed through the HTML5 canvas element.

With the release of Firefox 58, users will have the option to block websites’ requests to retrieve information through canvas, which is currently used as a cookie-less method of tracking users on the web. Websites using this technique extract data from HTML <canvas> elements silently.

As soon as the change will be in effect, Firefox will behave similarly with the Tor Browser, which is based on Firefox ESR (Extended Support Release). Tor implemented the feature about four years ago.

According to discussion on the Mozilla bug tracker, Firefox 58 should display a popup when accessing a website that attempts to use an HTML < canvas > element to extract information, just like Tor does in such situations. Users will have the option to block the site’s request.

As Sophos notes, many companies are using browser fingerprinting as means to track users online without providing them with a choice. The technique involves tracking the browser itself rather than cookies or other beacons, which can be blocked or deleted.

The fingerprinting operation usually involves passively gathering information such as browser version number, operating system details, screen resolution, language, installed plugins and fonts, and the like. The more elements are used for fingerprinting, the easier it is to single out one’s browser from another user’s, the security firm points out.

“In canvas fingerprinting your browser is given instructions to render something (perhaps a combination of words and pictures) on a hidden canvas element. The resulting image is extracted from the canvas and passed through a hashing function, producing an ID,” Sophos explains.

By providing complex instructions, one can produce enough variation between visitors to ensure canvas fingerprinting is highly efficient. The information gathered this way can be shared among advertising partners and used for the profiling of users based on the affiliated websites they visit.

While Firefox will become the first major browser to take a stance against canvas-based fingerprinting, add-ons that allow users to block this activity already exist, such as Electronic Frontier Foundation’s Privacy Badger.