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I have a little $7.20/y VPS, and the fingerprint for the ECDSA key has changed three times on me. This time, it happened shortly after I reinstalled the server (same day, in fact.) I always disable password login, and make sure you can't log in with passwords. After a while, I tried logging into my server, and I got the standard error telling me about the attack. I went ahead and disabled it, and was told that the host key for the domain I was logging into was different to the one for the IP. This nearly solidifies the idea of the attack by itself, but then I was prompted for a password. What tips do you have on stopping this? I always keep UFW enabled, and only allow SSH, and public/private key authentication is the only way to get into the server (without finding an exploit, ofc.)

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    Have you asked your hosting company? Maybe they shuffled your instance around and it broke the fingerprint. – schroeder Feb 5 '16 at 16:11
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    Reinstalling the server likely creates a new host key, so it should have changed immediately after reinstallation. – Michael Karcher Feb 5 '16 at 18:02
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    The host key is generated upon installation of openssh-server. If you reinstalled the server, the host key will change unless you manually restore the one you had previously. With that in mind, nothing in your post sounds particularly unusual. – tlng05 Feb 5 '16 at 19:27
  • Exactly what @tlng05 said. – cremefraiche Feb 8 '16 at 5:58
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    The IP address might have changed? – Wadih M. Mar 7 '16 at 7:37
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When a change in the fingerprint of the key is detected, it can mean a few things:

  • the key on the server genuinely changed - this is normal after an OS reinstallation and can happen after the sshd reinstallation
  • someone is posing as your server, trying a MitM attack. This is possible only with password-based authentication.

Your comment about a password-based authentication suddenly appearing out of the blue would hint about the second case.

As a side note, there is no added value for someone who already hacked into your server to change the ssh key as it will raise an alert.

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