In OpenStack Keystone before 29.0.3, any authenticated user holding role:reader on any project can list every project-scoped role assignment under any domain by passing a domain ID as scope.project.id with include_subtree to the GET /v3/role_assignments endpoint. The domain's project record has domain_id=null, causing the policy domain_id check to pass for any caller. With include_names, the response discloses the names and home-domain IDs of every user, group, project, and role involved. The literal "default" domain ID works against any deployment created with keystone-manage bootstrap. An attacker can harvest domain IDs from the response and repeat the query to map role assignments across the entire cloud. This is caused by misuse of "None" in list_role_assignments_for_tree.