Research papers in securing the RPL (Routing Protocol for Low-Power and Lossy Networks) in IoT focus on addressing the vulnerabilities and attack surfaces that arise due to the protocol’s operation in resource-constrained and distributed environments. Since RPL was originally designed for low-power devices and static topologies, its lightweight nature makes it susceptible to numerous routing attacks such as rank manipulation, sinkhole, selective forwarding, Sybil, wormhole, and denial-of-service (DoS). Research highlights that these attacks can degrade performance by increasing latency, dropping packets, draining energy, and compromising reliability, making security a critical requirement for IoT applications. To counter these threats, studies propose security-enhanced RPL variants that incorporate lightweight cryptographic mechanisms, trust and reputation-based routing, anomaly detection systems, intrusion-tolerant mechanisms, and blockchain-based distributed trust management. Some works extend RPL with secure Objective Functions that integrate authentication, integrity checks, and trust metrics into parent selection, while others propose hybrid security models that combine cryptography with machine learning to detect malicious behavior in real time. Comparative analyses with baseline RPL show that secure RPL protocols improve resilience against attacks but introduce trade-offs in energy consumption and control overhead. Application-driven research demonstrates how secure RPL is essential for mission-critical domains like smart healthcare, industrial IoT, intelligent transportation, and smart grids, where data integrity, availability, and trust are paramount. Collectively, this literature underscores that securing RPL is vital for the sustainable deployment of IoT systems and highlights future directions involving AI-driven adaptive defense, blockchain-assisted routing trust, and integration with zero-trust security models to protect large-scale, heterogeneous IoT networks.