Wireless Sensor Networks (WSN) is an emerging technology

and day by day it is attracting the attention of researchers

with its challenging characteristics and diversified application domain. The more researchers try to develop further

cost and energy efficient computing devices and algorithms

for WSN, the more challenging it becomes to fit the security

of WSN into that constrained environment. However, security is crucial to the success of applying WSN. So, familiarity

with the security aspects of WSN is essential before designing WSN system. This paper studies the security problems of

WSN based on its resource restricted design and deployment

characteristics and the security requirements for designing

a secure WSN. Also, this study documents the well known

attacks at the different layers of WSN and some counter measures against those attacks. Finally, this paper discusses on

some defensive measures of WSN giving focus on the key

management, link layer and routing security