A cluster of infected Linux servers has been discovered by Denis Sinegubko who is a security researcher.

“What we see here is a long awaited botnet of zombie web servers! A group of interconnected infected web servers with [a] common control center involved in malware distribution. To make things more complex, this botnet of web servers is connected with the botnet of infected home computer(s),” he wrote in a blog.

Each of the machines run a legitimate site according to the article at The Register. They are hacked to run a second webserver that serves malware.

Source: The Register