[Animats]

The protocol almost totally ignores security. They try to pass the buck downwards:

"As a transport protocol, MQTT is concerned only with message transmission and it is the implementer’s responsibility to provide appropriate security features. This is commonly achieved by using TLS."

and upwards:

"The Server MAY also use a security component to selectively authorize actions on the topic resource for a given Client."

This is a very weak security model. There's nothing like a distinction between "report body temperature" and "update pacemaker firmware".

[jonquark]

MQTT defines a wire protocol (that includes transmitting user & client id). Most MQTT brokers have a granular authorisation model so the topics that request (or just publish) a body temperature reading would allow a different set of users to publish than the topics that would cause pacemaker firmware to be updated.