Talk:Java Server Faces
I think the blog by EUxx missed some of the details of the implementation. Note that while the MAC and the IV are known, there is still a secret key known to the server used in the MAC that prevents the client from generating a new MAC. This is a fairly common pattern. You can see Inderjeet's response at: http://weblogs.java.net/blog/inder/archive/2005/05/securing_webapp.html
I think we should either take this section out, or expand on why its an issue.