Bluetooth 4.0 and 4.1 had a broken key exchange that was vulnerable to both passive and active attacks [1]. This could be remedied by a custom in-band or out-of-band key exchange, but I think it was rare for device manufacturers to go to those lengths.
The key exchange vulnerability was addressed in Bluetooth 4.2, which implements ECDH and is at least theoretically secure. [2]
The key exchange vulnerability was addressed in Bluetooth 4.2, which implements ECDH and is at least theoretically secure. [2]
[1] https://www.usenix.org/conference/woot13/workshop-program/pr...
[2] http://blog.bluetooth.com/everything-you-always-wanted-to-kn...