Radio Frequency Identification (RFID) and Wireless Sensor Networks (WSNs) are the two most prominent wireless technologies for implementing a complete smart environment for Internet of Things (IoT). Both technologies complement each other hence integration of both is used in a wide variety of applications like: traffic management, health, care monitoring systems, war-field, animal tracking, supply chain management etc. Security issue are of the most important concern and constraint of these kind of networks as both RFID and sensor networks have scarcity of resources which requires light computation based cryptography. In this work one cryptographic primitive of security, availability, is ensured through outlier detection mechanism. A threshold based anomaly detection mechanism is proposed which allows inliers only for secure communication, thus ensures availability.