Access control, sometimes called authorization, is how a web application grants access to content and functions to some users and not others. These checks are performed after authentication, and govern what ‘authorized’ users are allowed to do. Access control sounds like a simple problem but is insidiously difficult to implement correctly. A web application’s access control model is closely tied to the content and functions that the site provides. In addition, the users may fall into a number of groups or roles with different abilities or privileges. Developers frequently underestimate the difficulty of implementing a reliable access control mechanism. Many of these schemes were not deliberately designed, but have simply evolved along with the web site. In these cases, access control rules are inserted in various locations all over the code. As the site nears deployment, the ad hoc collection of rules becomes so unwieldy that it is almost impossible to understand. Many of these...