Stealing information like contacts or messages without the owner's knowledge. Bluebugging:

Full manipulation of the device, including placing calls, eavesdropping on conversations, and creating call-forwarding backdoors. How the Attack Works

Bluebugging is a hacking technique where an attacker exploits a discoverable Bluetooth connection to take control of a target device. It is more invasive than other Bluetooth attacks: Bluejacking: Sending unsolicited messages to nearby devices. Bluesnarfing: