    def render_POST(self, request):
        """
        Register with the Identity Server
        """
        send_cors(request)

        args = get_args(request, ('matrix_server_name', 'access_token'))

        result = yield self.client.get_json(
            "matrix://%s/_matrix/federation/v1/openid/userinfo?access_token=%s" % (
                args['matrix_server_name'], urllib.parse.quote(args['access_token']),
            ),
            1024 * 5,
        )
        if 'sub' not in result:
            raise Exception("Invalid response from homeserver")

        user_id = result['sub']
        tok = yield issueToken(self.sydent, user_id)

        # XXX: `token` is correct for the spec, but we released with `access_token`
        # for a substantial amount of time. Serve both to make spec-compliant clients
        # happy.
        defer.returnValue({
            "access_token": tok,
            "token": tok,
        })
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.