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I agree, hence my direct comment of malicious firmware… For me, the open question is, can one still write a malicious firmware on the ESP32 without the non documented opcodes?

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Yes. You can write whatever malicious firmware in a hardware you have physical access, with or without the undocumented opcodes. Not OTA though, unless there's a bug in the radio stack. Is not an open question.

HCI is an interface for the low level parts of the Bluetooth stack to exchange information with the higher levels. If you assume that higher level code is malicious, an OTA vulnerability is straightforward.

What would be the purpose of such firmware? The ESP32 is a complete SoC, the “firmware”, “OS”, and “application” are all the same binary.

So yes you could write a malicious “firmware” without using undocumented commands. But what would be the point? Said firmware already has complete execution privileges on the devices already, with the ability to read any memory it wants to, by virtue of said firmware being literally all the software running on the devices, and owning all of the memory.

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