It had to happen sooner or later; as Linux gains an ever-increasing foothold (Linux market share to reach 7% in 2008 ) in the market, it will become a viable target for criminal hackers. According to the U.S. Computer Emergency Readiness Team (CERT) in US-CERT Current Activity, attacks are already underway:
US-CERT is aware of active attacks against linux-based computing infrastructures using compromised SSH keys. The attack appears to initially use stolen SSH keys to gain access to a system, and then uses local kernel exploits to gain root access. Once root access has been obtained, a rootkit known as “phalanx2″ is installed.
Phalanx2 appears to be a derivative of an older rootkit named “phalanx”. Phalanx2 and the support scripts within the rootkit, are configured to systematically steal SSH keys from the compromised system. These SSH keys are sent to the attackers, who then use them to try to compromise other sites and other systems of interest at the attacked site.
For now, the attack is easily detected (though variants of the rootkit will likely change its behavior): The attack creates a directory “/etc/khubd.p2/” that is hidden from “ls,” but it can be entered with “cd /etc/khubd.p2″. Any directory named “khubd.p2,” regardless of its location, is hidden from “ls” but can be entered using “cd.” Additionally, “/dev/shm/” may contain files from the attack, so anything unusual in there is suspect. You can also try searching for hidden processes and checking the reference count in “/etc” against the number of directories shown by “ls”.
Check out the full article, “SSH Key-based Attacks” for complete details on risk mitigation and compromise response.