https://www.theregister.co.uk/2017/09/12/bluetooth_bugs_bedevil_billions_of_devices/

Security experts have long complained that complexity is the enemy of security, but the designers of the Bluetooth specification have evidently failed to pay attention.

Bluetooth is a wireless communication protocol for connecting devices over short ranges. It's used in mobile phones, wireless speakers, smartwatches, printers, and a variety of appliances, among other things.