Web applications are important, ubiquitous distributed systems whose current security relies primarily on server-side mechanisms. This paper makes the end-to-end argument that the client and server must collaborate to achieve security goals, to eliminate common security exploits, and to secure the emerging class of rich, cross-domain Web applications referred to as Web 2.0.

In order to support end-to-end security, Web clients must be enhanced. We introduce Mutation-Event Transforms: an easy-to-use client-side mechanism that can enforce even fine-grained, application-specific security policies, and whose implementation requires only straightforward changes to existing Web browsers. We give numerous examples of attractive, new security policies that demonstrate the advantages of end-to-end Web application security and of our proposed mechanism.