RPM is the packaging format used by Red Hat-based distributions like CentOS and CloudLinux.
SSHD is the daemon that implements the secure shell (SSH) protocol; SSH is is a program used to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another. It provides strong authentication and secure communications over insecure channels.
The ISC note said there was as yet no indication as to the actual attack vector. It said the rootkit was a trojanised library that linked with SSHD "and does a lot of nasty things to the system".
cPanel, an easy-to-use control panel for web hosts, has reported that it has been compromised by this rootkit.
In a message to its users, cPanel said: "cPanel, Inc. has discovered that one of the servers we utilize in the technical support department has been compromised. While we do not know if your machine is affected, you should change your root level password if you are not already using ssh keys. If you are using an unprivileged account with "sudo" or "su" for root logins, we recommend you change the account password. Even if you are using ssh keys we still recommend rotating keys on a regular basis."
The most common version of SSH used in Linux distributions is OpenSSH that is developed by the team that runs the OpenBSD operating system.
OpenBSD chief Theo de Raadt told iTWire: "I have not looked into it at all, because it is not a problem with our code. Someone built some nasty code, merged it as a change to a custom ssh, and fooled people into installing it."
He said it was hardly surprising that nobody yet knew the attack vector. "If I broke into someone's machine, and wanted to backdoor it, I would backdoor the sshd binary. ssh (port 22) is the most unfiltered port on the internet, after DNS. Other protocols are filtered much more, for instance, http or such."
De Raadt said it appeared that the rogue code was arriving in a library called libkeyutils.so. "Note that libkeyutils is not part of official ssh," he added. "The Linux people are adding all sorts of extra stuff to the 'ssh versions' they give to their customer base. Stuff that we've never looked at; it is their own 'value add', and it is not related to us."