@Limax Without knowing the standards by which the OSs were judged, it's impossible to say. DOS could be very secure in that it doesn't have attack vectors as you mentioned, but it could also be extremely unsecure as it doesn't have built in protections for the person just walking up and using it.
@mike I guess that's my beef with the author. He never explains why he ranks them the way he does other than user permissions. But legacy systems that are used the way they were originally intended are often more secure because their footprint is smaller and they have specialised uses. For instance, there are decades old mainframes that have never been breached or hacked. But they need to be replaced. Why?
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