Researchers from the Czech Republic say a self-generating botnet that steals mostly FTP credentials has been gradually growing in size, with up to 100,000 infected domains and some 1 million bots.

Avast Software's virus lab has been watching the so-called Kroxxu botnet over the past year, and this week disclosed some details on the password-stealing botnet. They say they still aren't sure how it makes its money.

"There are a number of ways they could be supporting themselves," Jiri Sejtko, head of virus research at the Avast Virus Lab, said in a statement. "The four most likely methods are through selling hacked space on infected servers, use of this malware to support the activities of other, more directly profitable malware, selling stolen credentials, or using keyloggers to spread other spam. But at this stage, it is more important for recognize this botnet than uncover its business plan."

What the researchers do know about the botnet is it's all server-based and spreads malware, such as keyloggers. When a password is stolen from an infected website, it then adds a script to the website content so it can alter files on the infected servers and spread to other severs. It relies heavily on redirection, with one connection spotted using 15 redirectors. Kroxxu also can swap roles among botnet elements: "Kroxxu's indirect cross infections are based on the fact that all parts being equal and interchangeable. If one part is used as an initial redirector, it may also be used as a final distribution part at the same or even a different time," said Sejtko.

It's not easy to remove Kroxxu from a server, and some websites have been infected for more than three months before Kroxxu was found, for example.

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