py-matrix-synapse -- malformed events may prevent users from joining federated rooms

Problem Description:

Affected Synapse versions assume that all events have an "origin" field set. If an event without the "origin" field is sent into a federated room, servers not already joined to the room will be unable to do so due to failing to fetch the malformed event.

Impact:

An attacker could cause a denial of service by deliberately sending a malformed event into a room, thus preventing new servers (and thus their users) from joining the room.