Forward Shibboleth Environment Variables to Tomcat via Apache

I am using Shibbolethv2.3 with Apache web server and Tomcat application server. I am using Apache as a reverse proxy using mod_proxy.so. I am not able to forward the Shibboleth environment variables from Apache to Tomcat. I am able to forward the attributes in the headers but as already mentioned in the wiki this approach is not safe.
I have tried forwarding the environment variables by the following directive :

SetEnv AJP_username ${username}

then at the Java side I can access the attribute by : request.getAttribute("username");

The strange thing here is that, I get a different value instead of the one set by Shibboleth. I get the Windows account name as a result. If I use any other attribute name, I get a null value.

I have searched a lot and have run out of options. Please guide me towards the right solution.

My setup details :

1. Shibboleth version : 2.3


2. OS : Windows XP SP3


3. Webserver : Apache 2.2



4. Application Server : Tomcat 6


5. Proxy module : mod_proxy.so