Let's call it "a window with a shitty lock" instead of a "back door", if it's an unintentional vulnerability. Then we can just use "back door [left open]" to mean something intentional. Or, you know "key under a rock in the garden" because only certain people know where it is. Actually, I think that's where "Window( with a )S(hitty lock)" 95 first got it's name.