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It's linked, and it's perimeter based security. I've also known it as egg-based security - once the shell breaks, you've got a big mess on your hands.

Ah, I get it now, thanks. Yes, it was linked, but the linked article didn't explain the metaphor either. I wasn't thinking of the candy, now I get it.

An M&M is a type of candy popular in the US with a hard outer shell and a softer interior. The author is using it metaphorically.

I think it's the author's name for perimeter based security

The analogy is pretty common, although the precise uses varies country to country. I called warm Smartie security, as I'm in the UK :)