Infected PCs Contain Average Of Three Different Types of Malware

An infected computer isn't just infested with one type of malware, but an average of three different malware families, according to new data gathered from antivirus firm Eset.

In scans of more than a half-million PCs in the past three months, Eset's free online scanner also found an average of 13 malware-infected files on each compromised machine. "These were not necessarily Eset customers. It was anyone who browses our site, which is why I thought this data was interesting," says Pierre-Marc Bureau, a researcher at Eset.

Bureau says such a high number of infected files was surprising. "A lot of people [at first] asked me to go back and make sure that data was accurate," he says; it was. Just which file was the original culprit can be difficult to ascertain, too, he says.

"It's hard to know which is the original malware versus the files it infected. If you had one media file, then all the media files are infected, and there's no way to differentiate between the initial one and the ones that became infected after. That's the problem," he says.

Bureau says it's all about the comeback of file-infecting viruses, which had been considered history until recently. Trojan downloader families, such as WMA/TrojanDownloader.GetCodec, spread quickly among multimedia files on a machine -- as soon as one infected music file is played, all others are infected, he says.

The Win32/Virut malware family works similarly, but with an extra punch: It infects executable files and then alters HTML files to insert iFrames on a site to infect anyone viewing that HTML file.

Bureau says the three malware-family average caught Eset by surprise, too. "I was not expecting to see that number now -- I expected to see it in about a year," he says.

That is due to a growing trend during the past six months of malware gangs installing one family of malware, and then another malware gang installing another. "That's why we see different malware families in the same system," he says. "I didn't know it was already that well-established [a method]," he says.

For example one gang specializes in infecting the PC, and another installs rogue antivirus. "Users usually get infected by a Trojan downloader and then rogue antivirus," he says. Rogue AV gangs pay other criminal gangs each time their phony software gets installed on the infected PC. This "pay per install" model for malware lets the bad guys focus on their "specialties" and contract to one another.

Meanwhile, Eset also reported today that among its own customers, 3.3 of all machines block at least one threat a day. That means 33 computers in a company with 1,000 computers will either try to open a malicious file on the Internet, receive a malicious email, or get hit with a worm, according to Eset, which gathered the data via its ThreatSense.Net monitoring system. "I think that number is quite high," Bureau says. "This means if your computer is not protected over a short period of time, it has lots of changes of being infected."

Eset's lab receives more than 100,000 new strains of malware daily, Bureau says.

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