I'm trying to design a solution where a ServiceStack server can just use an authentication cookie from ASP.NET. (In reality, it could be any cookie. It's just getting a session ID that it can lookup details using a back channel). The custom auth providers don't seem to be the right direction since they are based on credentials being sent. Instead, a GlobalRequestFilter made more sense to me. In there, I check the cookie, get the external session information, then set them to the ServiceStack session and set IsAuthenticated. This works fine in the request service as it has access to the session details that it needs. Fine so far.
The issue, is that when I decide to lock down services with the Authenticate attribute, it apparently runs the attribute prior to my filter so it always wants to redirect them to login. What is the recommended place to add my logic so it fires before the Authenticate attribute and validates properly?