Abstract: Security is a major threat in Wireless Sensor Networks (WSNs). These networks are increasingly used due to their broad range of important applications in both military and civilian domains. The WSNs are prone to several types of security attacks. Limited power and low memory are obstacles that make conventional security measures inappropriate for WSNs. Sensor nodes have limited capacities and are deployed in dangerous locations, therefore, they are vulnerable to different types of attacks, including wormhole, sinkhole and selective forwarding attacks. Security attacks are classified as data traffic and routing attacks. These security attacks could affect the most significant applications of WSNs, namely, military surveillance, traffic monitoring and healthcare. Therefore, there are different approaches to detecting security attacks on the network layer in WSNs. Reliability, energy efficiency and scalability are strong constraints on sensor nodes that affect the security of WSNs. Because sensor nodes have limited capabilities in most of these areas, selective forwarding attacks cannot be easily detected in networks. A compromised node selectively drops packets. A malicious node works in the same manner as any other node in the network. However, it tries to find sensitive messages and drop them before transferring packets to other nodes. In this study, we propose an approach to Selective Forwarding Detection (SFD). The approach has three layers: MAC pool IDs, rule-based processing and anomaly detection. It maintains the safety of data transmission between a source node and base station while detecting selective forwarding attacks. Furthermore, the approach is reliable, energy efficient and scalable.