There was no perimeter security here. The attacker did not first enter a private network and then pivot to MongoDB; he dialed MongoDB right from the internet. Had Mongo been un-authenticated on a private network it still might have been owned, but the bar would have been a lot higher.
Side note: everything that’s ever existed is “flawed,” it’s a weird word to use in the context of something you want to discredit, because then your alternative had better be “flawless” and it obviously isn’t.
There were several pieces here that conspired to produce the unfortunate end result, the blackmailing attacker exploiting the broken perimeter security was just the last piece.
There was perimeter security in this case. The user diligently configured a white-list only UFW firewall. That is their perimeter.
Docker diligently sidestepped that firewall, and in so doing exposed that this was a case of perimeter security. Because by bypassing that single external filter, the entire service was now vulnerable.
I guess this is hair-splitting semantics, but I think when most people say "perimeter security" in the context of a web production environment, they mean that things like DBs, message queues, and backend services share a private network with the servers that actually terminate user TCP connections.
Obviously with only perimeter security, those servers are soft targets to an attacker who compromises a frontend host. I am all for hardening the interior.
"Don't put stuff on the internet that doesn't need to be, even if you think it's secure, because it's probably complicated enough for you to be mistaken about that." This is a perimeter security philosophy, and also what OP needed. If anything the host-level firewall mishap seems closer to an application-level authz bypass than to a pivot across a "trustworthy" network.
...yet MongoDB's (default) configuration assumes there is. That's the big problem with perimeter security: applications offloading their security responsibility to other, possibly imaginary, parts of the system.
Side note: everything that’s ever existed is “flawed,” it’s a weird word to use in the context of something you want to discredit, because then your alternative had better be “flawless” and it obviously isn’t.