The invention is an improvement to conventional access control systems that use ACLs. The improvement comprises an Access Control Set, which is a data structure of access rules, ACLs, and other ACSs, and an improved object manager program that controls access to data objects according to the contents of an ACS. As in systems that rely exclusively on ACLs, every data object in the improved system includes a reference to an ACS that determines a user's access rights to the data object.