In Synapse before 0.31.2, unauthorised users can hijack rooms when there is no m.room.power_levels event in force.
{ "availability": "Available with Ubuntu Pro: https://ubuntu.com/pro", "ubuntu_priority": "low", "binaries": [ { "binary_version": "0.24.0+dfsg-1ubuntu0.1~esm1", "binary_name": "matrix-synapse" } ] }