FTA: "the discovery of a possible zero-day in widely distributed firmware" Is this CPU firmware? Or microcode? Or is it logic board firmware (UEFI base)? And i

FTA: "the discovery of a possible zero-day in widely distributed firmware" Is this CPU firmware? Or microcode? Or is it logic board firmware (UEFI base)? And if it's CPU firmware, is that replaceable or is it permanently baked into the CPU?