    def __init__(self, hs):
        super().__init__(hs)
        self.hs = hs
        self.is_mine_id = hs.is_mine_id
        self.http_client = hs.get_simple_http_client()

        self._presence_enabled = hs.config.use_presence

        # The number of ongoing syncs on this process, by user id.
        # Empty if _presence_enabled is false.
        self._user_to_num_current_syncs = {}  # type: Dict[str, int]

        self.notifier = hs.get_notifier()
        self.instance_id = hs.get_instance_id()

        # user_id -> last_sync_ms. Lists the users that have stopped syncing
        # but we haven't notified the master of that yet
        self.users_going_offline = {}

        self._bump_active_client = ReplicationBumpPresenceActiveTime.make_client(hs)
        self._set_state_client = ReplicationPresenceSetState.make_client(hs)

        self._send_stop_syncing_loop = self.clock.looping_call(
            self.send_stop_syncing, UPDATE_SYNCING_USERS_MS
        )

        hs.get_reactor().addSystemEventTrigger(
            "before",
            "shutdown",
            run_as_background_process,
            "generic_presence.on_shutdown",
            self._on_shutdown,
        )
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.