Abstract Today the WSN, playing a significant role in home security, habitat monitoring, battlefield-monitoring system, chemical industries, has a major security problem. Due to the deployment of WSN in open access environment and its resource constrained features such as limited bandwidth, limited computational ability, poor memory, low energy etc, WSN is vulnerable to various attacks such as Sybil attack, node clone attack, packet modification attack, node-dropping attack etc. In recent years, numerous data security protocols, such as symmetric encryption, key authentication, Key pre-distribution scheme, have proposed for securing the data transmitted between sensors and base station. This paper gives an overview of various adversary attacks in the various layers of WSN and defense mechanism used to avoid the mismanagement of sensors data. Keywords: Wireless sensor network, adversary attacks, defense mechanism, Key management. 1. Introduction Wireless sensor network comprises a large number of autonomous sensors, gateway node and base sensed data between the sensors, user and base station. WSN used in real-time application such as habitat monitoring system, battlefield, chemical industries, home security system etc 1. In WSN, sensor collects and processes environmental data. It transmits to another sensors and base station. The sensor that deployed in open access, harsh and remote unattended environment, is self-configuring and resource constrained by low battery power, low bandwidth, poor memory and limited computational capability 23. Sensors use a small event-driven OS called TinyOS. It consumes 4KB memory out of 8KB. The remaining 4KB used for the security purposes. In WSN, data security is a big problem due to the salient features of sensors. During data transmission, adversary node captures and misuses this valuable information. However, due to resource constrained and deployment of a sensor in an adversarial environment, WSN is highly vulnerable to various attacks such as clone attack, packet modification attack, packet dropping, packet misrouting, etc 45. Fig.1 Wireless sensor network For some application like military applications, WSN lacks in security because of the wireless communication used by sensors. In wireless communication, the eavesdropper can monitor the broadcast medium, intercepts and injects the malicious packets through the broadcast medium. Due to the deployment of a sensor in open access insecure environment, the adversary physically abducts the sensor, changes the cryptographic material through transanalysis and deploy as an authorized node. WSN is also vulnerable to resource consumption attack. In this attack, the intruder node sends a packet repeatedly to drain the energy and waste the bandwidth.