There is a superficial similarity between RBAC roles and traditional groups. As normally implemented, a group is a collection of users, rather than a collection of permissions, and permissions can be associated with both users and the groups to which they belong. The ability to tie permissions directly to users in a group-based mechanism can be regarded as a “loophole” that makes it difficult to control user-permission relationships. RBAC requires all access through roles, and permissions are connected only to roles, not directly to users. Another aspect of RBAC that distinguishes it from traditional group mechanisms is the concept of a session, which allows activation of a subset of roles assigned to a user. Core RBAC includes those systems with a robust group/ACL mechanism that supports the construction of a many-to-many relation among users and permissions.