Tricky new malware challenges vendors

A new piece of malware is using advanced techniques to foil antivirus software

A tricky malicious program has become more prevalent in spam, but experts don't know what its creators plan to do with it.

Many vendors are rating the malware -- called "Warezov," "Stration" and "Stratio" -- as a low risk. But they also say that it is tricky to deal with.

The malware is a mass-mailing worm that affects machines running Microsoft's Windows OS. When the malware infects a computer -- usually after the user has opened an attachment containing the worm in a spam e-mail -- it sends itself out again to other e-mail addresses found on the computer. The code is then capable of downloading new versions of itself as frequently as every 30 minutes from a batch of websites, said Mikko Hypponen, chief research officer at F-Secure, a security company in Helsinki.

Those new versions are created by a program on a server controlled by the hacker, Hypponen said.

In the past, malware has been known to create variations of itself, but the code to create those variations was contained inside the malware. So when a sample was obtained, security analysts could study it and identify potential new versions, he said.

Now, the hacker's program is compiling the code and rapidly churning out new versions, but analysts don't know how the new code is generated.

That characteristic is a headache for security software firms that issue special updates to their software to detect the malware. F-Secure alone has issued at least 150 signatures for the malware.

"It gets very complex to detect an attack like that because the code keeps changing," Hypponen said.

Security firm Sophos has detected some 300 versions of the malware. For October, the malware was one of the most common pieces of malicious code found in spam messages, said Carole Theriault, senior security consultant with Sophos.

Since infected computers look to other domains to receive updated code, F-Secure has worked with ISPs (Internet service providers) to shut down domains hosting the new variants. So far, nine of 10 domains have been shut down, Hypponen said.

Oddly, the malware doesn't appear to do anything yet on the victim's computers. It's estimated up to a few hundred thousand computers are infected, a sizable number but not quite on the scale of large malware problems from a few years ago, Hypponen said.

A hacker could be waiting to harness enough infected computers to start a denial-of-service attack or send spam or rent out the network to a spammer, Hypponen said.

"We hope to one day find out why they are doing this," Hypponen said. "We hope it's nothing too bad."

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