An access control boundary is a rule or mechanism that separates resources a user may access from those they may not. Authentication establishes or verifies identity; authorization evaluates what that identity is permitted to do. A successful login does not automatically grant access to every team, file, or system.
In a collaboration platform, team membership can act as a boundary around private messages and files. If an authorization flaw lets an authenticated user join an invite-only team without approval, the boundary may fail even though the login system works correctly. Defenders should apply security fixes, review membership and relevant join events, and ensure access checks are enforced independently of authentication.