Avoid putting rejected events in room state (#13723)

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/13723.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where previously rejected events could end up in room state because they pass auth checks given the current state of the room.

synapse/state/v2.py

@@ -577,6 +577,21 @@ async def _iterative_auth_checks(
                 if ev.rejected_reason is None:
                     auth_events[key] = event_map[ev_id]
 
+        if event.rejected_reason is not None:
+            # Do not admit previously rejected events into state.
+            # TODO: This isn't spec compliant. Events that were previously rejected due
+            #       to failing auth checks at their state, but pass auth checks during
+            #       state resolution should be accepted. Synapse does not handle the
+            #       change of rejection status well, so we preserve the previous
+            #       rejection status for now.
+            #
+            #       Note that events rejected for non-state reasons, such as having the
+            #       wrong auth events, should remain rejected.
+            #
+            #       https://spec.matrix.org/v1.2/rooms/v9/#rejected-events
+            #       https://github.com/matrix-org/synapse/issues/13797
+            continue
+
         try:
             event_auth.check_state_dependent_auth_rules(
                 event,