# UNDF: UNDF-2026-000000306
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1437,7 +1437,8 @@ class SyncHandler:
                 if event.membership == Membership.JOIN:
-                    user_ids_in_room = await self.store.get_users_in_room(room_id)
-                    if user_id in user_ids_in_room:
+                    # CWE-407 fix: get_users_in_room returns Sequence[str] (List); convert to set
+                    # for O(1) membership test instead of O(n) linear scan over all room members.
+                    user_ids_in_room = frozenset(await self.store.get_users_in_room(room_id))
+                    if user_id in user_ids_in_room:
                         mutable_joined_room_ids.add(room_id)