The attackers appear to be using stolen SSH keys to initially infiltrate a system, then leveraging an exploit to grant them root, or super-user, access, according to a US-CERT advisory. The goal is to drop a rootkit variant, known as phalanx2, on the compromised system that will steal additional SSH keys.
"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," the alert warned.
SSH keys offer secure and encrypted connections for clients wanting to access servers.
Linux administrators are encouraged to locate systems that use SSH keys and to ask users "to use the keys with passphrases or passwords to reduce the risk if a key is compromised."
If administrators detect a compromise, they should disable key-based SSH authentication on the affected systems and notify the key owners of the breach.
The current attacks may be related to a vulnerability in the OpenSSL library used to generate key pairs on Debian-based Linux distributions. The bug, reported in May, is caused by a weakness in the random number generator used to create SSL and SSH public and private cryptographic keys.