The team at McAfee Avert Labs located the malicious files, mostly MP3 or AVI in format, on popular peer-to-peer (P2P) websites, such as LimeWire, Craig Schmugar, a threat researcher, told SCMagazineUS.com on Wednesday.
The bogus files appear when users employ certain terms when searching for music and video, he said. Once the files are downloaded, users are directed to a website and alerted that they need to install an additional program, or codec, to play them.
That is where the malicious software kicks in. Installing the software and agreeing to the terms and conditions result in adware being placed on the victim's machine, Schmugar said.
Nearly a half-million home-users of McAfee VirusScan Online have the poisoned media files on their computers – the largest single outbreak of a single threat in three years, he said. Most victims have become infected in the past two days.
The criminals behind the scam have socially engineered the files so they appear like the real thing, Schmugar said. To that end, they use terms such as “theme godfather.mp3” to describe the contents and pad the files with null characters to make the sizes appear legitimate.
“These are media format files and people tend to trust them,” he said. “These file types are more trusted than typical vectors [to spread code].”
While potentially millions of users' machines worldwide contain the corrupted files, only a fraction of those individuals have actually installed the trojan by agreeing to download the fake media player, Schmugar said.
The attack could have been more destructive had the trojan forced its victims' machines to become part of a bot or sought identity theft through tactics such as password stealing, he added.
Minaxi Gupta, assistant professor of computer science at the Indiana University in Bloomington, who led a 2006 study on malware in P2P networks, said she and her team located 95 different types of malware, including downloaders, keyloggers and worms, in one-and-a-half months of research.
The problem gets so large because many P2P users unknowingly share rogue files with other users.
"Let's say I download something from a peer-to-peer network and maybe I don't execute it, and I put this data inside my shared network," she said. "It does nothing on my system, but it's in my shared directory.
"I think most malware that we see out there is not out of ill intent," Gupta added. "People get it and they spread it."
She told SCMagazineUS.com on Wednesday that file-sharing networks, such as LimeWire, perform limited checks to filter out malware. She and a group of graduate students are currently conducting additional research that will recommend, among other things, that P2P networks should add heuristics to detect malicious files.
“The only thing you can do [to protect yourself] is not join a P2P network and not download anything, or you can scan it to anti-virus when you get something,” she said. “Most people don't do that.”
In the case of the McAfee discovery, the files do not actually contain malware, so they would not be flagged during an anti-virus scan, Gupta said. However, the trojan would be discovered if the resulting codec -- supposedly required to play the file -- were run against a security check.
While mostly home users are affected by the attack, businesses should stay focused to the risks of file-sharing networks, Schmugar said.
“P2P has additional repercussions in corporate environments, most notably data leakage,” he said. “I think that people close to that issue are fully aware of the risks. It boils down to acceptable-use policies and whether or not companies want their employees downloading media files.”
Gartner Vice President and Senior Fellow John Pescatore told SCMagazineUS.com on Wednesday that businesses should, at least, run executables against known viruses.
"We basically tell businesses that there is no such thing as a business-quality peer-to-peer file-sharing type thing, he said. "There's vast quantifies of corrupted things on there."
A LimeWire representative could not be reached for comment.