Wireless Sensor Networks are easily attacked by jammers. Hence it is important to secure these networks from internal nodes that has a chance to become an adversary. In this paper, a novel approach is proposed for detection of node's maliciousness level for securing WSN's from jamming attacks in cluster-based sensor network. The proposed system works to detect the maliciousness level using two modules. Firstly, it protects the network from those internal nodes that are already announced as jammers. Secondly, it detects those nodes that are being becoming an adversary. The result of the simulation demonstrates that the proposed approach detects the maliciousness level enormously well and achieves high jammer detection rate and low false detection rate. © 2014 IEEE.