SSH key-based attacks

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.

Detection of phalanx2 as used in this attack may be performed as follows:

• “ls” does not show a directory “/etc/khubd.p2/”, but it can be entered with “cd /etc/khubd.p2”.
• “/dev/shm/” may contain files from the attack.
• Any directory named “khubd.p2” is hidden from “ls”, but may be entered by using “cd”.
• Changes in the configuration of the rootkit might change the attack indicators listed above. Other detection methods may include searching for hidden processes and checking the reference count in “/etc” against the number of directories shown by “ls”.

US-CERT encourages administrators to perform the following actions to help mitigate the risks:

• Proactively identify and examine systems where SSH keys are used as part of automated processes. These keys will typically not have passphrases or passwords.
• Encourage users to use the keys with passphrase or passwords to reduce the risk if a key is compromised.
• Review access paths to internet facing systems and ensure that systems are fully patched.

If a compromise is confirmed, US-CERT recommends the following actions:

• Disable key-based SSH authentication on the affected systems, where possible.
• Perform an audit of all SSH keys on the affected systems.
• Notify all key owners of the potential compromise of their keys.