McAfee has reported “the most significant malware outbreak in three years,” with more than 500,000 detections of a Trojan horse masquerading as a media file. A malicious MPEG or MP3 file which has been propagated through popular file-sharing sites has inflicted major damage.

McAfee researchers said they tracked more than half a million instances of the Trojan, Downloader-UA.h, on consumer PCs since Friday. They rated the threat a medium risk, and said no other malware has received that high of a risk rating in three years.

“This is one of the most prevalent pieces of malware in the past three years,” said Craig Schmugar, threat researcher at McAfee Avert Labs. “We have never before had a threat this significant that arrives as a media file.”

McAfee Labs say that the following filenames have been used: “preview-t-3545425-adult.mpg” ; “preview-t-3545425-changing times earth wind .mp3″ ; “preview-t-3545425-girls aloud st trinnians.mp3″ ; “preview-t-3545425-jij bent zo jeroen van den.mp3″ ; “t-3545425-lion king portugues.mpg” and “t-3545425-los padres de ella.mpg”

Click here to run a trojan scan if you believe you have downloaded Downloader-UA.h