Emerging storage cloud systems provide continuously available and highly scalable storage services to millions of geographically distributed clients. A secure access control mechanism is a crucial prerequisite for allowing clients to entrust their data to such cloud services. The seamlessly unlimited scale of the cloud and the new usage scenarios that accompany it pose new challenges in the design of such access control systems. In this paper we present a capability-based access control model and architecture appropriate for cloud storage systems that is secure, flexible, and scalable. We introduce new functionalities such as a flexible and dynamic description of resources; an advanced delegation mechanism and support for auditability, accountability and access confinement. The paper details the secure access model, shows how it fits in a scalable storage cloud architecture, and analyzes its security and performance. © 2011 SCPE.