I accidentally attempted to SSH into one of my servers from a device that did not contain my ssh key. I configure all of my servers to only allow authentication via cryptographic keys. Root ssh as well as password auth are disabled.
To my surprise, I was able to log in to my server with a password despite this. Baffled, I first tried some other servers. 2 of the 5 other servers I tried were accessabke via password.
After some swift investigation the culprit was found, a cloud-init ssh config in sshd_config.d/ with one line: password_authentication Yes.
So TLDR PSA....if you run a server in any type of virtualized environment, including a VPS, check your /etc/ssh/sshd_config.d/ folder. And more broadly, actually thoroughly test your ssh access to confirm everything is working as you intend it to.
This is good advice in general. Think of it like penetration testing. You really should verify what you can actually access remotely on a device and not assume you have any level of protection until you’ve tried it.
Log files can also contain signs of attack like password guessing. You should review these on a regular basis.