GHSA-hxmp-pqch-c8mm

### 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`).