A web application is constructed to process an intended sequence of requests. Failing to enforce the intended sequences can lead to request integrity (RI) attacks, wherein an attacker forces an application into processing an unintended request sequence. Cross-site-request forgeries (CSRF) and workflow violations are two classes of RI attacks. Enforcing the intended request sequences is essential for ensuring the integrity of the application. We describe a new approach for enforcing request integrity in a web application, and its implementation in a tool called Bayawak. Under our approach, the intended request sequences of an application are specified as a security policy, and a framework-level method enforces the security policy strictly and transparently without requiring changes in the application's source code. Our approach can be compared to operating system (OS) support for access control--access control is not built into the application, but based on OS level policy settings. We evaluated Bayawak using nine open source web applications. Our results indicate that our approach is effective against request integrity attacks and incurs negligible overhead.