As an emerging domain in high-performance computing, cluster computing is being widely used. However, there is little research work that comprehensively studies the security requirements in cluster-computing environments. Security mechanisms are important for cluster computing, especially when clusters are part of high-assurance systems. In this paper, we discuss security requirements for cluster-computing environments, with an emphasis on access control. A security solution is proposed based on the role-based access control (RBAC) model. Within an RBAC security model, roles are identified for different job assignments. Users are assigned roles according to their job functions. Required permissions are associated with each role. An example illustrates the coordination between the RBAC model and security sensors for cluster computing. RBAC is well suited to a cluster-computing environment and can be readily integrated with various security configurations.