WSNs applications include climate sensing and control in office buildings and home environmental sensing systems for temperature, light, moisture, and motion. WSNs suffer from many constraints, including low computation capability, small memory, limited energy resources, susceptibility to physical capture, and the use of insecure wireless communication channels. Wireless Sensor networks consist of hundreds or thousands of low cost, low power and self-organizing nodes which are highly distributed. Due to the reason that the sensor nodes are highly distributed, there is a need of security in the network. Security is an important issue nowadays in almost every network. There are some security issues and many attacks that need to be look around and work upon. Finally, this paper discusses on some defensive measures of WSN giving focus on the key management, link layer and routing security.