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By Jeremy Kirk, IDG News Service
July 18, 2008

A new kind of malicious software could pose a danger to Windows users who download music files on peer-to-peer networks.

The new malware inserts links to dangerous Web pages within ASF (Advanced Systems Format) media files.

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"The possibility of this has been known for a little while, but this is the first time we've seen it done," said David Emm, senior technology consultant for security vendor Kaspersky Lab.

Advanced Systems Format is a Microsoft-defined container format for audio and video streams that can also hold arbitrary content such as images or links to Web resources.

If a user plays an infected music file, it will launch Internet Explorer and load a malicious Web page that asks the user to download a codec, a well-known trick to get someone to download malware.

The actual download is not a codec but a Trojan horse, which installs a proxy program on the PC, Emm said. The proxy program allows hackers to route other traffic through the compromised PC, helping the hacker essentially cover their tracks for other malicious activity, Emm said.

The malware has wormlike qualities. Once on a PC, it looks for MP3 or MP2 audio files, transcodes them to Microsoft's Windows Media Audio format, wraps them in an ASF container, and adds links to further copies of the malware, in the guise of a codec, according to another security analyst, Secure Computing.

The ".mp3" extension of the files is not modified, however, so victims may not immediately notice the change, according to Kaspersky Lab.

Most savvy PC users are aware of the codec ruse, but the style of attack is still effective since many media players do need to receive updated codecs occasionally in order to play files.

"Users downloading from P2P networks need to exercise caution anyway, but should also be sensitive to pop-ups appearing upon playing a downloaded video or audio stream," Secure Computing said.

Users on a digital audio enthusiast site differed over the danger level of the malware.

"I never allow programs to choose which codecs I use to play back media," wrote JXL on the Hydrogen Audio forum. "I research it and get the codec bundles off of sites I know to be trustworthy and even then I still scan them and check to make sure they are what they are. I honestly don't feel that this malware has a very good chance of spreading fast."

But most users will probably think the prompt to download a codec is just routine business, wrote a user by the nickname of Citay on the same forum.

"I think that outside a minority of users who really know about all the dangers implied with Internet use, the vast majority of people have no idea that such a codec download could lead to a Trojan infection," Citay wrote.

Trend Micro calls the malware "Troj_Medpinch.a," Secure Computing named it "Trojan.ASF.Hijacker.gen," and Kaspersky calls it "Worm.Win32.GetCodec.a."

"since many media players do need to receive updated codecs occasionally in order to play files.
"

That's your problem right there.

When I get that message, I don't bother with the codec. I just delete that particular mp3, and look for another copy of the same song. They're usually not very hard to find.

My McAfee VirusScan deletes MP3 files that attempt to launch a web page like that. When I launched an MP3 that I downloaded with the infection it caught the script and advised me of the trojan and deleted the file.

mcafee is a horrible anti virus im amazed it detected it at all get avg or avira from free-av.com

You shouldn't need to update a codec to play a particular media file, unless said codec already loaded is way out of date. but, alas for an mp3, it shouldn't need to download any...


If your concerned about malware, I'd suggest this program as your primary defense against malware of this sort.

http://www.malwarebytes.org

The malwarebytes antimalware scanner is what your interested in.