All complex devices, like CPUs, GPUs, microcontrollers, have tons of undocumented debug features, without exception. The only difference between devices is whe

All complex devices, like CPUs, GPUs, microcontrollers, have tons of undocumented debug features, without exception. The only difference between devices is whether such debug features remain accessible to users or they are completely disabled after production. In this specific case, as well explained in TFA, the debug commands do not really provide any additional exploitable capabilities for a malicious programmer, especially because the Bluetooth controller is already hosted on the same CPU as the potentially malicious application, so if that could use the debug commands it could already do anything it wants, like sending arbitrary Bluetooth packets. The Bluetoth driver also can already do anything it wants on the shared CPU, so if it had bugs or backdoors that can be triggered by received Bluetooth packets the existence of these debug commands does not change anything. Unlike this case where there is nothing to criticize in the reply of ExpressIf or on what they have done, an example of atrocious handling of the undocumented debug features has been provided by Apple, whose devices had for several years, until the end of 2023, a backdoor created by non-disabled debug registers, which allowed a total bypassing of the memory protection, enabling (in conjunction with some bugs in Apple system libraries) complete remote control of an iPhone, and which has been exploited for the remote and undetectable spying of some iPhone owners (discussed on HN at the time of the public revealing of the CVEs).