Wireless Sensor Networks (WSNs) have recently emerged as an important research topic. Due to the enormous number of sensor nodes and the constrained resources, specific research challenges can be identified with respect to security. Almost all available commercial and research sensor nodes are equipped with ZigBee transceiver chips, and thus making ZigBee the de-facto standard in WSN communication. Since Joshua Wright's KillerBee Framework was released with its focus on exploring and exploiting the security of ZigBee networks, non security-hardened WSNs increase the risk of being vulnerable against certain attacks such as simple association flooding and packet replay attacks. We propose an anomaly-based approach intrusion detection system (IDS) optimized for ZigBee-based WSN to protect ZigBee-based WSN nodes against KillerBee supported attacks. We describe the KillerBee attack procedure and propose an approach of guarding a ZigBee transceiver. Based on an extended sensor node/network simulation and analysis framework, we demonstrate furthermore how our anomaly-based detection engine can thwart attacks on a ZigBee transceiver.