Recently there was a panicked scrambling after the announcement by [Tarlogic] of a ‘backdoor’ found in Espressif’s popular ESP32 MCUs. Specifically a backdoor on  the Bluetooth side that would give a lot of control over the system to any attacker. As [Xeno Kovah] explains, much about these claims is exaggerated, and calling it a ‘backdoor’ is far beyond the scope of what was actually discovered.

To summarize the original findings, the researchers found a number of vendor-specific commands (VSCs) in the (publicly available) ESP32 ROM that can be sent via the host-controller interface (HCI) between the software and the Bluetooth PHY. They found that these VSCs could do things like writing and reading the firmware in the PHY, as well as send low-level packets.

The thing about VSCs is of course that these are a standard feature with Bluetooth controllers, with each manufacturer implementing a range of these for use with their own software SDK. These VSCs allow for updating firmware, report temperatures and features like debugging, and are generally documented (except for Broadcom).

Effectively, [Xeno] makes the point that VSCs are a standard feature in Bluetooth controllers, which – like most features – can also be abused. [Tarlogic] has since updated their article as well to distance themselves from the ‘backdoor’ term and instead want to call these VSCs a ‘hidden feature’. That said, if these VSCs in ESP32 chips are a security risk, then as [Xeno] duly notes, millions of BT controllers from Texas Instruments, Broadcom and others with similar VSCs would similarly be a security risk.