If an MQTT v5 client connects to Eclipse Mosquitto versions 1.6.0 to 1.6.4 inclusive, sets a last will and testament, sets a will delay interval, sets a session expiry interval, and the will delay interval is set longer than the session expiry interval, then a use after free error occurs, which has the potential to cause a crash in some situations.

In versions 1.6 to 2.0.11 of Eclipse Mosquitto, an MQTT v5 client connecting with a large number of user-property properties could cause excessive CPU usage, leading to a loss of performance and possible denial of service.

In Eclipse Mosquitto version 1.6 to 2.0.10, if an authenticated client that had connected with MQTT v5 sent a crafted CONNECT message to the broker a memory leak would occur, which could be used to provide a DoS attack against the broker.