In Eclipse Mosquitto 1.4.14, a user can shutdown the Mosquitto server simply by filling the RAM memory with a lot of connections with large payload. This can be done without authentications if occur in connection phase of MQTT protocol.
https://www.debian.org/security/2018/dsa-4325
https://mosquitto.org/blog/2018/02/security-advisory-cve-2017-7651-cve-2017-7652/
https://lists.debian.org/debian-lts-announce/2018/06/msg00016.html
https://lists.debian.org/debian-lts-announce/2018/03/msg00037.html