Security Service as Proxy

I've been tasked with creating a Security Proxy service. The idea is that if the backend security provider changes there is no impact on the main application. This ideally is what the backend security provider is for, but I have been tasked with creating a seperate service which will affectively be a proxy to the backend 开发者_JAVA百科security provider.

I don't want to have to write a complete security module to do something that is already done by a dozen services. I want to be able to set up a service that can be updated if needs be.

I am wondering if anyone knows of a solution which can take care of this with minimal coding/configuration?

Any help would be useful, if you want more information please comment and I'll try and enrich as best I can.

[Front end is Tomcat Web Application written in Java (and GWT), Spring Security is preferable]

[Backend is SiteMinder (at the moment)] http://www.ca.com/us/internet-access-control.aspx

[I have been looking at CAS but wanted to ask a learned community before deciding how best to proceed]