In this paper, the authors study about MANET (Mobile Ad hoc NETwork) and security attacks present in MANET. MANETs are powerless against different types of attacks that are present at different layers. Black hole attack is an external attack and present at network layer. Black hole attack present itself having the shortest route to the destination, but actually it doesn't have any information about the route to destination and after getting the packets it will drop all the packets. Cooperative black hole attack is an attack in which two or more black hole nodes are working in a group.