The MQTT protocol provides a good exemplar for understanding the need for finer-grained access controls. As a publish/subscribe protocol, MQTT allows clients to write and read topics. Not all clients will have permissions to write all topics. Not all clients will have permissions to read all topics either. Indeed, controls must be put in place that restrict the permissions of clients at the topic level.
This can be achieved in an MQTT broker by keeping an access control list that pairs topics with authorized publishers and authorized subscribers. The access controls can take as input the client ID of the MQTT client or, depending on the broker implementation, the username ...
