Security for IoT, mobile, and web applications is an extremely important topic that deserves entire books dedicated to it. Each solution has its own security requirements, and it is very important to consider all of them when developing each component of the solution.
If we use MQTT to publish values that are neither confidential nor critical for other applications, our only concern may be to keep control over the maximum number of subscribers to topics to make sure that the messages are always available. This way, we can avoid the failure of MQTT in delivering messages to a huge number of subscribers.
However, most of the times, we won't be working on a solution that can share the data with the entire world without limitations and doesn't need to care about data confidentiality and integrity in addition to data availability. Imagine that we are working on a solution that includes a mobile app that allows users to control a huge drone. If the drone flies the wrong...