Acegi Filter Does Not Work in WebSphere 6.1.0.3

It seems there is a bug in WAS that was introduced with 6.1.0.3. I found this forum post on the subject.

Apparently, WAS looks for a resource in the WAR that matches the URL before allowing the servlet filters a chance to process the URL. Despite the fact that the design of the Servlet Filter mechanism is to allow web applications more control over their URLs and how they are accessed.

This affects all servlet filters not just Acegi's. IBM has had similar issues in the past like this one and this one. You'd think they could get it right after a few tries.

Some of the wording in these bug reports and the reply from IBM posted on that Springframework thread appear to indicate that IBM sees nothing wrong with this behavior. I disagree. The servlet specification does not place any restrictions on how a Servlet filter can transform a request URL so it stands to reason that no check for the existence of the referenced resource should be performed until the filter are processed.