Is it a bug or is it a backdoor?

is exposed to a UEFI bug that can be exploited to disable firmware write-protection.

If the claims made by Dmytro Oleksiuk at Github are correct, an attacker can “disable flash write protection and infect platform firmware, disable Secure Boot, [and] bypass Virtual Secure Mode (Credential Guard, etc.) on Windows 10 Enterprise.”

The reason Oleksiuk believes other vendors are also vulnerable is that the buggy code is inherited from Intel. He writes that the SystemSmmRuntimeRt was copied from Intel reference code.

Source: Lenovo scrambling to get a fix for BIOS vuln

Also confirmed on HP pavilions