filter out m.room.aliases from the CS API until a better solution is specced (#6878)

We're in the middle of properly mitigating spam caused by malicious aliases being added to a room. However, until this work fully lands, we temporarily filter out all m.room.aliases events from /sync and /messages on the CS API, to remove abusive aliases. This is considered acceptable as m.room.aliases events were never a reliable record of the given alias->id mapping and were purely informational, and in their current state do more harm than good.

changelog.d/6878.feature

@@ -0,0 +1 @@
+Filter out m.room.aliases from the CS API to mitigate abuse while a better solution is specced.

synapse/visibility.py

@@ -122,6 +122,13 @@ def filter_events_for_client(
         if not event.is_state() and event.sender in ignore_list:
             return None
 
+        # Until MSC2261 has landed we can't redact malicious alias events, so for
+        # now we temporarily filter out m.room.aliases entirely to mitigate
+        # abuse, while we spec a better solution to advertising aliases
+        # on rooms.
+        if event.type == EventTypes.Aliases:
+            return None
+
         # Don't try to apply the room's retention policy if the event is a state event, as
         # MSC1763 states that retention is only considered for non-state events.
         if apply_retention_policies and not event.is_state():