Security in wireless sensor networks is critical due to its way of open communication. In this study we have proposed a technique based on a sensor node having alternate path knowledge and 2-hop acknowledgement mechanism to detect adversary nodes which perform selective forwarding and dropping attacks. In selective forwarding attack nodes on the forwarding path refuses to transfer packets selectively. The proposed approach starts with network initialization where every node decides the list of

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... arent nodes through which Sink can be reached with equal distance. Each node chooses a parent node among selected parents to forward the data and establishes pairwise keys with 2-hop parent nodes. During data forwarding, child forwards the packet to 1-hop distance parent, handles acknowledgement from 2-hop distance node and decides the number of packets forwarded and dropped based on successful and unsuccessful transactions. Every node sends a transaction report containing observations on the parent via alternate path to Sink at a particular interval of time called an evaluation period. Sink identifies the malicious node by comparing report received from each node with number of data packets received. Simulated the algorithm in NS-3 and performance analysis compared with other recently proposed approach. Simulation results show that proposed method detects the malicious nodes efficiently and early.