A remote Matrix user with malicious intent, sharing a room with Synapse instances before 1.105.1, can dispatch specially crafted events to exploit a weakness in how the auth chain cover index is calculated. This can induce high CPU consumption and accumulate excessive data in the database of such instances, resulting in a denial of service.
Servers in private federations, or those that do not federate, are not affected.
Server administrators should upgrade to 1.105.1 or later.
One can: - ban the malicious users or ACL block servers from the rooms; and/or - leave the room and purge the room using the admin API
If you have any questions or comments about this advisory, please email us at security AT element.io.