Wireless Sensor Network (WSN) has become an emerging technology of wireless communication and computing. Wireless Sensor Network is a self-configured, infrastructure-less wireless network which comprises of a few to large number of spatially distributed autonomous sensor nodes and a base station. The function of the base station is to interface user and network. A sensor node combines hardware and software. The main deployment of sensor network is sensing, data processing and transmitting data. WSN has different application in enemy intrusion detection in the battlefield in the military application, medical field, atmospheric disaster, agricultural and industrial applications. In Wireless sensor network each node is a battery-operated low power device and operated in ad-hoc principle, which indicates network power is mainly dependent on rate of energy consumption. Ad-hoc networking nature of WSN allows the attacker for different types of attacks from passive eavesdropping to active attacks. As WSN requires hop by hop routing to transport the packets to the destination, any intermediate node acting maliciously can drop, modify or misguide the traffic traversing through it. Confidentiality, authenticity, integrity, data freshness and quality of service (QoS) are the most important issues for wireless sensor nodes. An attacker may collect and destroy sensitive information if the transmission is not properly encrypted. However, avoiding collision and providing cooperation among the nodes during the transmission are done by using medium access control protocols. This paper focuses on the security threats on the resource restricted design and deployment characteristics along with the requirements to design a secured WSN system. In this paper wormhole attacks, black hole attacks, Sybil attacks, hello flood attack, denial of service attacks and private attacks are investigated. In this paper different link layer attacks are also discussed. Not only does this document about the popular attacks in different layers of WSN, but also provides some remedies against these attacks