Cloud Computing has emerged as a major information and communications technology trend and has been proved as a key technology for market development and analysis for the users of several field. The practice of computing across two or more data centers separated by the Internet is growing in popularity due to an explosion in scalable computing demands. However, one of the major challenges that faces the cloud computing is how to secure and protect the data and processes the data of the user. The security of the cloud computing environment is a new research area requiring further development by both the academic and industrial research associations. While cloud-bursting is addressing this process of scaling up and down across data centers .To provide secure and reliable services in cloud computing environment is an important issue. One of the security issues is how to reduce the impact of denial-of-service (DoS) attack or distributed denial-of-service (DDoS)in this environment. In this paper we survey several aspects of cloud computing and the security concerns.