Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. A malicious user on a Synapse homeserver X with permission to create certain state events can disable outbound federation from X to an arbitrary homeserver Y. Synapse instances with federation disabled are not affected. In versions of Synapse up to and including 1.73, Synapse did not limit the size of "invite_room_state", meaning that it was possible to create an arbitrarily large invite event. Synapse 1.74 refuses to create oversized "invite_room_state" fields. Server operators should upgrade to Synapse 1.74 or newer urgently.