Access drift is the gradual mismatch between the permissions people or systems are supposed to have and the access they actually retain. It can happen when roles change, accounts are copied between platforms, temporary permissions are not removed, or policies differ across connected services.
During a migration, old and new systems may run in parallel, making it easier for permissions to become inconsistent or overlooked. Excess access can let an account reach data or functions it no longer needs, while missing access can disrupt work. Security teams reduce drift by documenting roles, reviewing permissions regularly, removing obsolete accounts and privileges, and comparing access settings across systems before and after changes.