Cloud computing is a revolutionary concept that has brought a paradigm shift in the IT world. This has made it possible to manage and run businesses without even setting up an IT infrastructure. It offers multi-fold benefits to the users moving to a cloud, while posing unknown security and privacy issues. User authentication is one such growing concern and is greatly needed in order to ensure privacy and security in a cloud computing environment. This paper discusses the security at different levels viz. network, application and virtualization, in a cloud computing environment. A security framework based on one-time pass key mechanism has been proposed. The uniqueness of the proposed security protocol lies in the fact, that it provides security to both the service providers as well the users in a highly conflicting cloud environment.