In Synapse before 0.31.2, unauthorised users can hijack rooms when there is no m.room.power_levels event in force.
{
"cpe": "cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*",
"source": "CPE_FIELD",
"extracted_events": [
{
"introduced": "0"
},
{
"fixed": "0.31.2"
}
]
}