Hi,

I run a few Rightscale CentOS AMI based instances on Amazon EC2. Two months back I found that our SSHD security is compromised( I had added host.allow and host.deny for ssh). So I created new instances and done an IP based ssh that allows only our IPs through AWS Firewall(ec2-authorize) and chnaged the ssh 22 default port to some other port but two days back I found I was not able to login to the server and when I tried on 22 port the ssh got connected and I found that sshd_conf was changed and when I tried to edit sshd_config I found root had no write permission on the file. So I tried a chmod and it said access denied for 'root' user. This is very strange. I checked secure log and history and found nothing informative. I have PHP, Ruby On Rails, Java, Wordpress apps running on these server.

This time I did a chkrootkit scan and found nothing. I renamed the /etc/ssh folder and reinstalled openssh through yum. I had faced this on 3 instances on CentOS(5.2, 5.4) I have instances on Debian as well those working fine. Is this a CentOS/Rightscale issue. Guys, what security measures I should take to prevent this.

Please support me this is very critical.

Thanks