A lightweight MQTT substitute designed for resource-constrained IoT environments is the Message Queuing Telemetry Transport for Sensor Networks (MQTT-SN) protocol. Although it offers efficiency benefits in terms of power consumption, message size, and transport, it is vulnerable to various cyberthreats because it lacks built-in security measures. The four key areas of healthcare, smart homes, industrial IoT, and vehicle systems are the focus of this paper’s thorough literature review on MQTT-SN communication security flaws. In comparison to conventional MQTT vulnerabilities, it analyzes attack surfaces such as denial-of-service vectors, authentication flaws, and the lack of encryption. Additionally, the review assesses countermeasures that have been suggested in recent studies, such as intrusion detection systems (IDS), lightweight cryptographic protocols, secure MQTT-SN extensions, and authentication based on Physical Unclonable Functions (PUFs). In order to help researchers and practitioners create reliable and scalable security frameworks for MQTT-SN-enabled IoT applications, this work attempts to identify open challenges and emerging trends.