Abstract. Websites today routinely combine JavaScript from multiple sources, both trusted and untrusted. Hence, JavaScript security is of paramount importance. A specific interesting problem is information flow control (IFC) for JavaScript. In this paper, we develop, formalize and implement a dynamic IFC mechanism for the JavaScript engine of a production Web browser (specifically, Safari's WebKit engine). Our IFC mechanism works at the level of JavaScript bytecode and hence leverages years of industrial effort on optimizing both the source to bytecode compiler and the bytecode interpreter. We track both explicit and implicit flows and observe only moderate overhead. Working with bytecode results in new challenges including the extensive use of unstructured control flow in bytecode (which complicates lowering of program context taints), unstructured exceptions (which complicate the matter further) and the need to make IFC analysis permissive. We explain how we address these challenges, formally model the JavaScript bytecode semantics and our instrumentation, prove the standard property of terminationinsensitive non-interference, and present experimental results on an optimized prototype.
Web browsers routinely handle private information. Owing to a lax security model, browsers and JavaScript in particular, are easy targets for leaking sensitive data. Prior work has extensively studied information flow control (IFC) as a mechanism for securing browsers. However, two central aspects of web browsers -the Document Object Model (DOM) and the event handling mechanism -have so far evaded thorough scrutiny in the context of IFC. This paper advances the state-of-the-art in this regard. Based on standard specifications and the code of an actual browser engine, we build formal models of both the DOM (up to Level 3) and the event handling loop of a typical browser, enhance the models with fine-grained taints and checks for IFC, prove our enhancements sound and test our ideas through an instrumentation of WebKit, an inproduction browser engine. In doing so, we observe several channels for information leak that arise due to subtleties of the event loop and its interaction with the DOM.
Preventing implicit information flows by dynamic program analysis requires coarse approximations that result in false positives, because a dynamic monitor sees only the executed trace of the program. One widely deployed method is the no-sensitive-upgrade check, which terminates a program whenever a variable's taint is upgraded (made more sensitive) due to a control dependence on tainted data. Although sound, this method is restrictive, e.g., it terminates the program even if the upgraded variable is never used subsequently. To counter this, Austin and Flanagan introduced the permissive-upgrade check, which allows a variable upgrade due to control dependence, but marks the variable "partially-leaked". The program is stopped later if it tries to use the partially-leaked variable. Permissive-upgrade handles the dead-variable assignment problem and remains sound. However, Austin and Flanagan develop permissive-upgrade only for a two-point (low-high) security lattice and indicate a generalization to pointwise products of such lattices. In this paper, we develop a non-trivial and non-obvious generalization of permissive-upgrade to arbitrary lattices. The key difficulty lies in finding a suitable notion of partial leaks that is both sound and permissive and in developing a suitable definition of memory equivalence that allows an inductive proof of soundness
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