java-topology/docs/tickets/synapse-0002-sync-get-users-in-room-sequence-linear-scan.md

synapse-0002: CWE-407 in synapse — sync handler get_users_in_room Sequence linear scan

Severity: HIGH File: synapse/handlers/sync.py:1439 Pattern:

for room_id, event in mem_last_change_by_room_id.items():   # O(R) rooms changed
    ...
    if event.membership == Membership.JOIN:
        user_ids_in_room = await self.store.get_users_in_room(room_id)  # returns Sequence[str] (List)
        if user_id in user_ids_in_room:                                 # O(U) linear scan
            mutable_joined_room_ids.add(room_id)

Complexity: O(R × U) — for each of R rooms that changed membership, scans all U users in the room Fix: set(await self.store.get_users_in_room(room_id)) or restructure to avoid the membership check entirely Speedup: 100–10000× at large room size (U=10000 users, R=10 rooms → 100000 comparisons vs 10 hash lookups) Hot path: _generate_sync_entry_for_rooms() — called on every sync request when membership changes occur