Cloud computing security is comprised of elements drawn from computer security, network security and information security. It encompasses policies, technologies, and controls employed to protect data, applications, and the cloud computing infrastructure. The security aspects of cloud computing are examined from the point of view of its inherent vulnerabilities with regard to availability, user authentication and privacy and trust. Recent events such as the April 21, 2011 collapse of the Amazon Elastic Compute Cloud (EC2) are used to illustrate these vulnerabilities and to suggest measures that can be implemented to improve cloud security.