The U.S. Computer Emergency Readiness Team (CERT) has issued a warning for what it calls "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, US-CERT said in a note on its current activity site.
From the advisory:
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.
Phalanx, which dates back to 2005, is a self-injecting kernel rootkit designed for the Linux 2.6 branch. It allows an attacker to hide files, processes and sockets and includes a tty sniffer, a tty connectback-backdoor, and auto injection on boot.