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Undocumented Commands Found In Bluetooth Chip Manufactured in China Used By a Billion Devices.
(www.tarlogic.com)
Source Link Privacy.
Tarlogic Security has detected a backdoor in the ESP32, a microcontroller that enables WiFi and Bluetooth connection and is present in millions of mass-market IoT devices. Exploitation of this backdoor would allow hostile actors to conduct impersonation attacks and permanently infect sensitive devices such as mobile phones, computers, smart locks or medical equipment by bypassing code audit controls.
Update: The ESP32 "backdoor" that wasn't.
Tasmota is another option if they have your specific device in their list. Otherwise you have to do some debugging to figure out what gpio or i2c address to use.