Cloud computing is considered to be the computing paradigm that offers numerous flexible and consistent services using virtualization technology that is used in the next generation of the data centers. Cloud computing by means of its capacity, resilience and cost minimization that provides the capability to share resources in a pervasive and transparent way, also it has the ability to perform procedures that meet different needs. The increasing use of cloud services poses threat to security of data and system infrastructure. Security attacks as a result of loss of availability of cloud services can have more significant impact. A critical review of one of those attack Denial of service, their impact and the mechanism to defend is the scope this work.