py-matrix-synapse -- DoS on Federation API

Matrix developers reports:

A malicious or poorly-implemented homeserver can inject malformed events
into a room by specifying a different room id in the path of a /send_join,
/send_leave, /invite or /exchange_third_party_invite request.
This can lead to a denial of service in which future events will not be
correctly sent to other servers over federation.
This affects any server which accepts federation requests from untrusted
servers.