Euacalyptus, OpenNebula and Nimbus are three major open-source cloud-computing software platforms. The overall function of these systems is to manage the provisioning of virtual machines for a cloud providing infrastructure-as-a-service. These various open-source projects provide an important alternative for those who do not wish to use a commercially provide cloud. This is a fundamental concept in cloud computing, providing resources to deliver infrastructure as a service cloud customers, making users have to buy and maintain computing resources and storage. In other hand, cloud service providers to provide better resources and facilities customers need to know they are using cloud infrastructure services. In this end, we intend to security threats in the cloud layer and then to analyse the security services in cloud computing infrastructure as a service to pay.