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Apparently I installed that thing in 2006 and I last updated it in 2016, then I quit updating it for some reason that I totally forgot. Probably laziness...
It's been running for quite some time and we kind of forgot about it in the closet, until the SSH tunnel we use to get our mail outside our home stopped working because modern openssh clients refuse to use the antiquated key cipher I setup client machines with way back when any longer.
I just generated new keys with a more modern cipher that it understands (ecdsa-sha2-nistp256) and left it running. Because why not 🙂
Most 'hackers' are just mid tier (mediocre) IT level types who rely on existing exploits floating around in the wild. It'd probably be hard to find any still in circulation for such an old system.
We're not talking about some punch card COBOL machine he jimmy rigged with network access, it's an old Debian Linux box with SSH enabled.
It's not like Metasploit would have a tough time finding unpatched vulnerabilities for it...
What makes it a even bigger target is the fact that it is a mail server
Unless it's for SMTP only, it's probably a back end sever to some other front facing box, or service, that has IP addresses whitelisted for email.
I'm pretty sure I read one of his comments elsewhere talking about tunneling everything over SSH, so I assume that's what he meant, but I could be mistaken.
Regardless, using an EOL distro as an internet facing SSH server that's 8 years behind on SSH updates, is probably a bad idea.