(Sorry, third time -- last one, promise, just giving it a subject line!)
OK, a second machine hosted at the same hosting company has also apparently been hacked. Since 2 of out of 3 machines hosted at that company have now been hacked, but this hasn't happened to any of the other 37 dedicated servers that I've got hosted at other hosting companies (also CentOS, same version or almost), this makes me wonder if there's a security breach at this company, like if they store customers' passwords in a place that's been hacked. (Of course it could also be that whatever attacker found an exploit, was just scanning that company's address space for hackable machines, and didn't happen to scan the address space of the other hosting companies.)
So, following people's suggestions, the machine is disconnected and hooked up to a KVM so I can still examine the files. I've found this file: -rw-r--r-- 1 root root 1358 Oct 21 17:40 /home/file.pl which appears to be a copy of this exploit script: http://archive.cert.uni-stuttgart.de/bugtraq/2006/11/msg00302.html Note the last-mod date of October 21.
No other files on the system were last modified on October 21st. However there was a security advisory dated October 20th which affected httpd: http://mailinglist-archive.com/centos-announce/2011-10/00035-CentOSannounce+... https://rhn.redhat.com/errata/RHSA-2011-1392.html
and a large number of files on the machine, including lots of files in */ usr/lib64/httpd/modules/* and */lib/modules/2.6.18-274.7.1.el5/kernel/* , have a last-mod date of October 20th. So I assume that these are files which were updated automatically by yum as a result of the patch that goes with this advisory -- does that sound right?
So a couple of questions that I could use some help with:
1) The last patch affecting httpd was released on October 20th, and the earliest evidence I can find of the machine being hacked is a file dated October 21st. This could be just a coincidence, but could it also suggest that the patch on October 20th introduced a new exploit, which the attacker then used to get in on October 21st? (Another possibility: I think that when yum installs updates, it doesn't actually restart httpd. So maybe even after the patch was installed, my old httpd instance kept running and was still vulnerable? As for why it got hacked the very next day, maybe the attacker looked at the newly released patch and reverse-engineered it to figure out where the vulnerabilities were, that the patch fixed?)
2) Since the */var/log/httpd/* and /var/log/secure* logs only go back 4-5 weeks by default, it looks like any log entries related to how the attacker would have gotten in on or before October 21st, are gone. (The secure* logs do show multiple successful logins as "root" within the last 4 weeks, mostly from IP addresses in Asia, but that's to be expected once the machine was compromised -- it doesn't help track down how they originally got in.) Anywhere else that the logs would contain useful data?