SoK: Exploring Current and Future Research Directions on XS-Leaks through an Extended Formal Model

A web visit typically consists of the browser rendering a dynamically generated response that is specifically tailored to the user. This generation of responses based on the currently authenticated user, whose authentication credentials are automatically included via cookies in all (including cross-site) requests, have led to a multitude of issues. Through cross-site leaks (XS-Leaks), an adversary can try to circumvent the same-origin policy and extract information about responses, which in turn can reveal potentially sensitive information about the user. As research on this class of vulnerabilities only recently gained traction, and the attacks affect many different components of the web platform, the intrinsic characteristics and underlying causes remain largely unexplored.

In this paper we present an abstraction of XS-Leaks attacks and introduce an extended formal model that we use to reason about the cause of different leaks and which strategies the various defense mechanisms employ to defend against them. Furthermore, we provide a classification method for current attacks, and, guided by our model, propose a methodology to comprehensively detect new XS-Leak issues, or indicate their absence. Furthermore, we analyze the current defenses and identify gaps that still require further research to provide extensive solutions for sites that rely on cross-site interactions. Finally, we explore how XS-Leak defenses are currently deployed and which challenges website owners are still facing. As a first step towards facilitating the deployment of XS-Leak defenses, we introduce LeakBuster, a dynamic web interface that provides web developers with suggestions based on the insights provided throughout this paper.