How to find out if my server is compromised?

Question

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?

Answer 1

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

http://serverfault.com/questions/120839/linux-backdoors-i-should-be-wary-of

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.

Answer 2

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

Answer 3

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.