Authentication in WCF REST Routing Scenario

Question

I developed a WCF REST Router which redirects incoming messages to WCF REST targets by inspecting the message, changing the message.Headers.To uri and forwarding it to the target.

Now I would like to use Basic and NTLM authentication. The authentication of the client against the router works fine. But I need the router to authenticate to the target with the users credentials. But I don't know the credentials in the router, specially when using NTLM auth.