XSinator.com: From a Formal Model to the Automatic Evaluation of Cross-Site Leaks in Web Browsers

Cross-Site Leaks (XS-Leaks) describe a client-side bug that allows an attacker to collect side-channel information from a cross-origin HTTP resource. They are a significant threat to Internet privacy since simply visiting a web page may reveal if the victim is a drug addict or leak a sexual orientation. Numerous different attack vectors, as well as mitigation strategies, have been proposed, but a clear and systematic understanding of XS-Leak' root causes is still missing. Recently, Sudhodanan et al. gave a first overview of XS-Leak at NDSS 2020. We build on their work by presenting the first formal model for XS-Leaks. Our comprehensive analysis of known XS-Leaks reveals that all of them fit into this new model. With the help of this formal approach, we (1) systematically searched for new XS-Leak attack classes, (2) implemented XSinator.com, a tool to automatically evaluate if a given web browser is vulnerable to XS-Leaks, and (3) systematically evaluated mitigations for XS-Leaks. We found 14 new attack classes, evaluated the resilience of 56 different browser/OS combinations against a total of 34 XS-Leaks, and propose a completely novel methodology to mitigate XS-Leaks.