Mobile Adhoc Networks (MANET) are infra structure-less network made up of many mobile nodes and these nodes communicate through a wireless and infrastructure less medium. Security becomes a major challenge for MANET due to its dynamic characteristics, changing topologies and random mobility. This makes MANET susceptible to a wide verity of attacks. Also, a majority of routing protocols in MANETS are influenced by different classes of attack. This paper focuses on the study of the effects of the cooperative blackhole attack in MANET, how to identify such attacks in ZRP, which is hybrid routing protocol. In this paper, firstly, an attempt has been made to study the effect of cooperative attacks over DSR (Reactive), DSDV (Proactive) and Hybrid (ZRP). Afterward, ZRP is modified by introducing two additional parameters so that effect of cooperative attacks is reduced in ZRP. The modification is based on fuzzy approach which helps in identifying cooperative black hole in ZRP is proposed. Several experiments have been performed to validate the improvement in performance of proposed modified ZRP.