    def setUp(self):
        self.reactor = ThreadedMemoryReactorClock()
        self.hs_clock = Clock(self.reactor)
        self.homeserver = setup_test_homeserver(
            self.addCleanup, http_client=None, clock=self.hs_clock, reactor=self.reactor
        )
A web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a Redirect. This simplifies phishing attacks.