The existence of the Via C3 "backdoor" was actually documented in the official datasheet, along with the correct MSR bit to enable/disable it. See page A-10 in

The existence of the Via C3 "backdoor" was actually documented in the official datasheet, along with the correct MSR bit to enable/disable it. See page A-10 in appendix A: http://datasheets.chipdb.org/VIA/Nehemiah/VIA%20C3%20Nehemiah%20Datasheet%20R113.pdf http://datasheets.chipdb.org/VIA/Nehemiah/VIA%20C3%20Nehemia... Apparently the researchers either couldn't find a copy or didn't notice that part.