The word mobile indicates the meaning of moving and the word ad hoc indicates the meaning of temporary or do not have any type of constant infrastructure, therefore mobileadhoc-networks implies the networks that are temporary and allows the nodes to move within the network without any centralized administration. In this paper, the concern is on security issues found in the MANET. Then certain criteria for the security of mobile-adhoc-network and some major types of attacks are discussed which are present in it. Mainly this paper is targeted on the attacks of jellyfish and their some types. Due to some special features of MANET such as hopby-hop communications, dynamic topology and simple and fast setup, this network has suffered lots of issues like issues in security, routing and some clustering issues. Hence, this paper tries to work by focusing on mainly for maintaining the security in MANET and some TCP-based attacks it is facing. .