Antipassback

Antipassback is a security mechanism in access control systems designed to prevent credential sharing and tailgating. It ties entry and exit events to a single credential so that a card used to enter cannot be used by another person to gain access until the original user has exited (or the system otherwise resets the state).

Typical antipassback setups use paired entry and exit readers at a door. When a user presents their credential at the entry reader, access is granted and the system marks that credential as active inside. To exit, the same credential must be presented at the exit reader; upon successful exit, the credential's state is cleared. If a second person attempts to pass with a different credential while the first one is still inside, the system denies the second credential or records a violation.

Antipassback can be implemented as hard antipassback, using physical interlocks or dual-door configurations to prevent passage, or soft antipassback, relying on software rules and alarms. Some systems offer global antipassback, applying the rule across multiple doors within a facility; local antipassback applies to a single door or zone. Time-based or role-based exceptions can also be configured.

While antipassback improves security and auditability, it can increase user friction and complicate door operation. Proper implementation requires reliable reader placement, synchronized clocks, and a defined timeout window; misconfiguration can lead to user lockouts or unwanted denials. It is not a universal solution and should be combined with other security controls.