Matrix Synapse before 0.33.3.1 allows remote attackers to spoof events and possibly have unspecified other impacts by leveraging improper transaction and event signature validation.
https://matrix.org/blog/2018/09/06/critical-security-update-synapse-0-33-3-1/
https://github.com/matrix-org/synapse/issues/3796#event-1833126269