Cloud computing is a model to provide convenient, on-demand access to a shared pool configurable computing resources. In cloud computing, IT-related capabilities are provided as services, accessible without requiring detailed knowledge of the underlying technologies, and with minimal management effort. The great savings promised by the cloud are however offset by the perceived security threats feared by users. This paper gives an overview of cloud computing, and discusses related security challenges. We emphasize that although there are many technological approaches that can improve cloud security, there are currently no one-size-fits-all solutions, and future work has to tackle challenges such as service level agreements for security, as well as holistic mechanisms for ensuring accountability in the cloud.