TLS Beyond the Broker: Enforcing Fine-grained Security and Trust in Publish/Subscribe Environments for IoT

TL;DR

This paper proposes an enhanced publish/subscribe system for IoT that enforces fine-grained security levels, ensuring messages are only forwarded over connections meeting specified security requirements, addressing security gaps in traditional brokers.

Contribution

It introduces a method to enforce security levels in MQTT brokers, allowing clients to specify and control the security of message forwarding based on connection security.

Findings

Prototype implementation for MQTT demonstrated feasibility.

Overhead measurements show acceptable performance impact.

Enhanced security control for IoT publish/subscribe systems.

Abstract

Message queuing brokers are a fundamental building block of the Internet of Things, commonly used to store and forward messages from publishing clients to subscribing clients. Often a single trusted broker offers secured (e.g. TLS) and unsecured connections but relays messages regardless of their inbound and outbound protection. Such mixed mode is facilitated for the sake of efficiency since TLS is quite a burden for MQTT implementations on class-0 IoT devices. Such a broker thus transparently interconnects securely and insecurely connected devices; we argue that such mixed mode operation can actually be a significant security problem: Clients can only control the security level of their own connection to the broker, but they cannot enforce any protection towards other clients. We describe an enhancement of such a publish/subscribe mechanism to allow for enforcing specified security levels of publishers or subscribers by only forwarding messages via connections which satisfy the desired security levels. For example, a client publishing a message over a secured channel can instruct the broker to forward the message exclusively to subscribers that are securely connected. We prototypically implemented our solution for the MQTT protocol and provide detailed overhead measurements.