Rendered private: Making GLSL execution uniform to prevent WebGL-based browser fingerprinting

Browser fingerprinting, a substitute of cookies-based tracking, extracts a list of client-side features and combines them as a unique identifier for the target browser. Among all these features, one that has the highest entropy and the ability for an even sneakier purpose, i.e., cross-browser fingerprinting, is the rendering of WebGL tasks, which produce different results across different installations of the same browser on different computers, thus being considered as fingerprintable. Such WebGL-based fingerprinting is hard to defend against, because the client browser executes a program written in OpenGL Shading Language (GLSL). To date, it remains unclear, in either the industry or the research community, about how and why the rendering of GLSL programs could lead to result discrepancies. Therefore, all the existing defenses, such as these adopted by Tor Browser, can only disable WebGL, i.e., a sacrifice of functionality over privacy, to prevent WebGL-based fingerprinting. In this paper, we propose a novel system, called UNIGL, to rewrite GLSL programs and make uniform WebGL rendering procedure with the support of existing WebGL functionalities. Particularly, we, being the first in the community, point out that such rendering discrepancies in state-of-the-art WebGL-based fingerprinting are caused by floating-point operations. After realizing the cause, we design UNIGL so that it redefines all the floating-point operations, either explicitly written in GLSL programs or implicitly invoked by WebGL, to mitigate the fingerprinting factors. We implemented a prototype of UNIGL as an open-source browser add-on (https://www.github.com/unigl/). We also created a demo website (http://test.unigl.org/),i.e., a modified version of an existing fingerprinting website, which directly integrates our add-on at the server-side to demonstrate the effectiveness of UNIGL. Our evaluation using crowdsourcing workers shows that UNIGL can prevent state-of-the-art WebGL-based fingerprinting with reasonable FPSes.