@rustleg - Just so you know and as I stated before in a previous comment on this thread, the most probable reason these level 5 updates are now shown is due to one Ubuntu developer's offhand comment (a rather old comment in a mailing list) about the so-called lack of security in Linux Mint because kernel, Xorg and Firefox (believe it or not) were not updated by default, a comment that was somehow picked up by the media and blown way out of proportion. Of course, the "comment monsters"
had a field day with this as well.
So now these level 5 updates are now shown by default but are not checked. Before version 17, level 4 and 5 updates were not enabled by default but could easily be
if the user chose to do so however, they'd be checked by default if they were enabled.
So now we see the reason Linux Mint initially chose not to enable level 4 and 5 updates by default in the first place. Your comment is the perfect example of why this was so. After years of Linux Mint versions without so much as a peep about this so called "lack-of-security", there was a sudden uproar about something that didn't actually exist in the first place since, in the past, the Mint devs always announced to it's users and the world in general when a level 5 update(s) needed to be applied due to a valid security vulnerability or a major bug.
Unfortunately, there's no middle ground here. Either Mint shows these level 5 updates or not. Personally, I don't think there's a right answer here.
"Humph. Choice, it is the quintessential Linux delusion, simultaneously the source of it's greatest strength, and it's greatest weakness." (All apologies to The Architect)