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I have a linux VPS and I received a complaint that my server was scanning on port 22 some other network. How do I find out if it was compromised or not?

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I answered a question earlier today that has recommendations in this area:

If you suspect that it may have been one of your users and your default shell is bash, you can grep through the .bash_history. For example:

grep nmap /home/*/.bash_history

It's notable that your users can modify the history unless you've introduced methods to make it more difficult.

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Well, if you WERE scanning.... what is the doubt?

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I'm not scanning dude. That is the problem lol – Jene Hackman Mar 9 '10 at 22:15
How you know? Take a measurement from outside (firewall, router in front, wahtever). – TomTom Mar 10 '10 at 6:13

You have to think outside the box. As in, you can't really trust what's inside the box.

For instance, get your provider to monitor outgoing traffic. Traffic you can't explain => assume the server has been compromised.

Get an image of the hard drive (extracted using trusted binaries) and run forensics.

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