In Eclipse Mosquitto 1.4.15 and earlier, a Memory Leak vulnerability was found within the Mosquitto Broker. Unauthenticated clients can send crafted CONNECT packets which could cause a denial of service in the Mosquitto Broker.
https://www.debian.org/security/2018/dsa-4325
https://usn.ubuntu.com/4023-1/
https://lists.debian.org/debian-lts-announce/2018/09/msg00036.html