Cloud computing is an emergent technology that provides all the services to the users such as SaaS, PaaS and IaaS which makes life simpler. Cloud gains its limelight with its effective elastic services. This paper makes the user to feel convenient but there arise so many security breaches as cloud's central theme is outsourcing. The data processed on cloud are usually outsourced and hence the user is worried about the data loss or modification. All other security mechanisms are not suitable for cloud because of its decentralized and distributed architecture.