Mozilla will bring anti fingerprinting technique ‘letterboxing’ for Firefox

Mozilla will bring anti fingerprinting technique ‘letterboxing’ for Firefox

Now, Mozilla will give its Firefox browser a method to hide the size of the window to advertising agencies.

A new anti-fingerprinting technique, called Letterboxing, is exploited only when the advertising code tries to repatriate the dimension data. The aim of this project is to improve Firefox privacy protection features by relying on those of experiments conducted by the developers of the Tor browser. According to the reports, this is a part of the Tor Uplift project started in July 2016.


The new anti-fingerprinting technique adds gray spaces to the sides of the web page whenever a user resizes the browser window and removes after the resize operation is over. It will mask the window’s actual dimensions by adding space to width and height at multiples of 200px and 100px while resizing. So the gray space will be added at the top, bottom, left, or right of the current web page. It delays the loading of actual page content on the newly-resized window; long enough to trick the tracker codes or scripts to read the incorrect window dimensions.


The researches show that the dimensions of a browser window can act as a big source of finger-printable data that can be used by advertising networks.

Those advertising networks can use browser window sizes to create user profiles and track users as they resize their browser and move across new URLs and browser tabs. To maintain users privacy on the Internet, it needs to protect this window dimension data continuously even if users resize or maximize their window or enter fullscreen.


By enabling anti-fingerprinting features in the browser could break down certain services, including captchas, whose verification often fails with no workaround until the protection system is disabled.


Letterboxing feature is currently available in Firefox Nightly. And most probably in May this year, its stable version will be available for all users with the next version Firefox 67.


Stock photo from dennizn


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