In Eclipse Mosquito before and including 2.0.5, establishing a connection to the mosquitto server without sending data causes the EPOLLOUT event to be added, which results excessive CPU consumption. This could be used by a malicious actor to perform denial of service type attack. This issue is fixed in 2.0.6

In Mosquitto before 2.0.16, a memory leak occurs when clients send v5 CONNECT packets with a will message that contains invalid property types.

In Mosquitto before 2.0.16, excessive memory is allocated based on malicious initial packets that are not CONNECT packets.

The broker in Eclipse Mosquitto 1.3.2 through 2.x before 2.0.16 has a memory leak that can be abused remotely when a client sends many QoS 2 messages with duplicate message IDs, and fails to respond to PUBREC commands. This occurs because of mishandling of EAGAIN from the libc send function.

In Eclipse Mosquitto versions 2.07 and earlier, the server will crash if the client tries to send a PUBLISH packet with topic length = 0.

In Eclipse Mosquitto 1.5.0 to 1.6.5 inclusive, if a malicious MQTT client sends a SUBSCRIBE packet containing a topic that consists of approximately 65400 or more '/' characters, i.e. the topic hierarchy separator, then a stack overflow will occur.