fix(federation): reject cross-room auth events on inbound PDUs#227
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An inbound event's auth_events were accepted without checking they belong to the room (receipt rule 3.5), so a peer could cite a foreign-room m.room.power_levels/member as an auth event to fake authority it doesn't hold here. Now reject any resolved auth event whose room_id differs from the event's own; m.room.create is exempt (it's bound to the room by check_auth rule 2 via the room_id↔create-id relation).
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An inbound event's
auth_eventswere accepted without checking they belong to the room (receipt rule 3.5), so a peer could cite a foreign-roomm.room.power_levels/member as an auth event to fake authority it doesn't hold here. Now reject any resolved auth event whoseroom_iddiffers from the event's own;m.room.createis exempt (already bound to the room bycheck_authrule 2 via the room_id↔create-id relation).Test: an event citing another room's member event as an auth event is rejected.
(Scope note: this PR originally also added the restricted-join authoriser-server signature check — auth rule 5.2.1. CI surfaced that it rejected legitimate restricted joins whose
join_authorised_via_users_servercame through in an unexpected form, so that half was reverted for separate investigation. This PR is the safe, tested cross-room-auth-events half.)