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That's the gotcha that can bite you: if you're sharing internal and external sites via a split horizon nginx config, and it's accessible over the public internet, then the actual IP defined in DNS doesn't actually matter.
If the attacker can determine that secret.local.mydomain.com is a valid server name, they can request it from nginx even if it's got internal-only dns by including the header of that domain in their request, as an example, in curl like thus:
curl --header 'Host: secret.local.mydomain.com' https://your.public.ip.here -kAdmittedly this requires some recon which means 99.999% of attackers are never even going to get remotely close to doing this, but it's an edge case that's easy to work against by ACLs, and you probably should when doing split horizon configurations.
But the attacker should know the internal and the external DNS. If the internal DNS doesn't have any SSL certificate on its name, it's impossible to discover.
By the way, I always suggest to reach services through VPN and use something like Cloudflare tunnel for services that must be public.
P.s. Shouldn't public and private DNS be inverted in your curl example?
Nope, that curl command says 'connect to the public ip of the server, and ask for this specific site by name, and ignore SSL errors'.
So it'll make a request to the public IP for any site configured with that server name even if the DNS resolution for that name isn't a public IP, and ignore the SSL error that happens when you try to do that.
If there's a private site configured with that name on nginx and it's configured without any ACLs, nginx will happily return the content of whatever is at the server name requested.
Like I said, it's certainly an edge case that requires you to have knowledge of your target, but at the same time, how many people will just name their, as an example, vaultwarden install as vaultwarden.private.domain.com?
You could write a script that'll recon through various permuatations of high-value targets and have it make a couple hundred curl attempts to come up with a nice clean list of reconned and possibly vulnerable targets.