The web is now being used as a general platform for hosting distributed applications like wikis, bulletin board messaging systems and collaborative editing environments. Data from multiple applications originating at multiple sources all intermix in a single web browser, making sensitive data stored in the browser subject to a broad milieu of attacks (cross-site scripting, crosssite request forgery and others). The fundamental problem is that existing web infrastructure provides no means for enforcing end-to-end security on data. To solve this we design an architecture using mandatory access control (MAC) enforcement. We overcome the limitations of traditional MAC systems, implemented solely at the operating system layer, by unifying MAC enforcement across virtual machine, operating system, networking and application layers. We implement our architecture using Xen virtual machine management, SELinux at the operating system layer, labeled IPsec for networking and our own label-enforcing web browser, called FlowwolF. We tested our implementation and find that it performs well, supporting data intermixing while still providing end-to-end security guarantees.