It is notoriously difficult to program a solid web application. Besides addressing web interactions, state maintenance, and whimsical user navigation behaviors, programmers must also avoid a minefield of security vulnerabilities. The problem is twofold. First, we lack a clear understanding of the new computation model underlying web applications. Second, we lack proper abstractions for hiding common and subtle coding details that are orthogonal to the business functionalities of specific web applications.

It is well recognized that JavaScript can be exploited to launch browser-based security attacks. We propose to battle such attacks using program instrumentation. Untrusted JavaScript code goes through a rewriting process which identifies relevant operations, modifies questionable behaviors, and prompts the user (a web page viewer) for decisions on how to proceed when appropriate. Our solution is parametric with respect to the security policy-the policy is implemented separately from the rewriting, and the same rewriting process is carried out regardless of which policy is in use. Be-sides providing a rigorous account of the correctness of our solution, we also discuss practical issues including policy management and prototype experiments. A useful by-product of our work is an operational semantics of a core subset of JavaScript, where code embedded in (HTML) documents may generate further document pieces (with new code embedded) at runtime, yielding a form of self-modifying code.