    def test_it_works_with_explicit_external_host(self, app, monkeypatch):
        with app.test_request_context():
            monkeypatch.setattr('flask.request.host_url', 'http://example.com')
            result = _validate_redirect_url('http://works.com',
                                            _external_host='works.com')
            assert result is True
            monkeypatch.undo()
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.