Wireless Sensor Networks (WSN) is one of the fastest rising emerging technologies that find widespread use in various applications comprising of military, health, agriculture, habitat etc. Seen as sensor network deployed at sites which can be considered as remote and hostile, the technology is seriously faced with challenges to the network and functional security at the cost of their inherent limitations in energy capacity and computing power. In this paper we have delved upon and summarized earlier research on security challenges poised to WSNs, classified the threats and then presented a generic WSN security model keeping in line with the intended security targets to be met. We have also tried to give a realistic theoretical analysis of our WSN security model against these threats.