Bluetooth is a short-range wireless link used for phone-based access credentials, locks and some sensors. Its security depends on the pairing, the encryption of the connection and how the reader proves the phone is genuine. Because range is only a few meters, Bluetooth suits door-side credentials better than site-wide networking.
A phone in a pocket is now a common door credential, and the wireless link that makes it work has its own failure modes. Those are separate from the access control logic behind the door.
How a phone credential works
The phone stores a credential and presents it over Bluetooth Low Energy when the reader is close. Some systems require a deliberate gesture; others read at a distance. The link should be encrypted and bound to one device, so a copied credential file is less useful on other hardware.
Weak points to watch
Relay attacks, where two devices extend the link beyond its real range, are the classic risk, and distance estimation is the usual mitigation. Pairing modes and factory default codes on wireless locks matter too. So does firmware, because a lock that never updates keeps the security it shipped with.