On Sunday night computers infected with a well-known virus began downloading a spam tool used by hackers to send unwanted email.
Virus writers part of the infamous Bagle spam gang began sending a spamming tool to thousands of hacked computers, said Mikko Hypponen, chief research officer for F-Secure, a security company based in Helsinki.
If a computer is infected with the Bagle virus, a hacker can download other malicious programs to the system. In turn, those programs can send out spam to other PCs without the knowledge of the user.
Once a computer is under their control, virus writers can upgrade the malicious software they have installed. Last night, systems infected with Bagle variants were downloading the tool from a server in Slovakia hosting a real-estate website.
"This is the way virus writers can upgrade the infected machines," Hypponen said. "It’s like (Microsoft’s) Windows update for viruses."
The download link was buried within the website, and it’s unlikely the owners had any idea it was being used. Last night, the link was cut off, Hypponen said.
But within hours, a French site hosted in the US was hosting a link with the malware, Hypponen said. The ISP (internet service provider) hosting that site has been contacted, but so far the link remains active, he said.
F-Secure uses automated tools to poll URLs used by virus writers to host bad malware, Hypponen said.
About 99 percent are decoys, set up to throw off attempts to track them down. But when a site suddenly becomes active, Hypponen said efforts are made to contact ISPs to shut them down.