The Eclipse Mosquitto broker up to version 1.4.15 does not reject strings that are not valid UTF-8. A malicious client could cause other clients that do reject invalid UTF-8 strings to disconnect themselves from the broker by sending a topic string which is not valid UTF-8, and so cause a denial of service for the clients.
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
https://bugs.eclipse.org/bugs/show_bug.cgi?id=532113
http://docs.oasis-open.org/mqtt/disallowed-chars/v1.0/disallowed-chars-v1.0.pdf