Denial of service attack via incorrect parameters in Matrix Synapse

Impact

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.

Patches

Issue is resolved by https://github.com/matrix-org/synapse/pull/8776.

Workarounds

Homeserver administrators could limit access to the federation API to trusted servers (for example via federation_domain_whitelist).