In Synapse before 0.31.2, unauthorised users can hijack rooms when there is no m.room.power_levels event in force.
{
"cwe_ids": [],
"nvd_published_at": "2018-06-14T21:29:00Z",
"github_reviewed": true,
"severity": "HIGH",
"github_reviewed_at": "2023-07-22T00:05:35Z"
}