Wireless Sensor Networks (WSNs) are typically formed by the collaboration of the large amount of partial sensor nodes, which are connected through a wireless medium. In wireless sensor network, security is an essential aspect because of its usage in applications like monitoring, tracking, controlling, surveillance etc. Secure communication is extremely crucial in delivering vital information accurately and on the time through resource constraint sensor nodes. In this paper, our contribution is threefold. Firstly, we have summarized the network layer routing attacks on WSNs. Secondly, we have provided a taxonomy of secure routing protocols of WSNs. Thirdly, we have provided a qualitative comparison of existing secure routing protocols. Results show that most of the existing secure routing schemes are not very efficient due to various reasons like high-energy consumption, and large communication overhead.