Sensor nodes, when deployed to form Wireless sensor network operating under control of central authority i.e. Base station are capable of exhibiting interesting applications due to their ability to be deployed ubiquitously in hostile & pervasive environments. But due to same reason security is becoming a major concern for these networks. Wireless sensor networks are vulnerable against various types of external and internal attacks being limited by computation resources, smaller memory capacity, limited battery life, processing power & lack of tamper resistant packaging. This survey paper is an attempt to analyze threats to Wireless sensor networks and to report various research efforts in studying variety of routing attacks which target the network layer.