Well, it appears that it would be as safe as any other vulnerable server behind port knocking. Although there's still the possibility that the backdoor is doin

Well, it appears that it would be as safe as any other vulnerable server behind port knocking. Although there's still the possibility that the backdoor is doing something we aren't aware of yet. So yeah, it's as "safe" as you consider port knocking to be. That said, just make sure you aren't using the backdoored versions. I don't see how you can reasonably skip this step.