I think IInternetSession::RegisterNameSpace() is exclusive. That is, if anybody use it to register a namespace, then the old handler of this namespace will be replaced. Then there is a problem if several IE plugins use PassthroughAPP at the same time: only the last registered one could take effect! Then I thought if we can get the registered handler before we register ourselves, and then pass all the request to this old handler, then the problem can be avoid. But is there any interface can do this? Or any other good idea? Thanks!