CVE-2020-26890

Matrix Synapse before 1.20.0 erroneously permits non-standard NaN,
Infinity, and -Infinity JSON values in fields of m.room.member events,
allowing remote attackers to execute a denial of service attack against the
federation and common Matrix clients. If such a malformed event is accepted
into the room’s state, the impact is long-lasting and is not fixed by an
upgrade to a newer version, requiring the event to be manually redacted
instead. Since events are replicated to servers of other room members, the
impact is not constrained to the server of the event sender.