Santa Clara: Nearly half a million people have been caught out by a booby-trapped media file, says security firm McAfee.

The fake file poses as a music track, short video or movie and has been widely seeded on file-sharing networks to snare victims.

The outbreak is the largest McAfee has seen in the last three years.

The trojan has been widely distributed on the eDonkey and Limewire file-sharing networks. Those running the fake file get bombarded with pop-up ads and risk compromising the safety of their PC.

The file has many names and is written in different languages to trick people into downloading it. The titles make the file appear to be music tracks, pornography and full versions of popular movies.

Anyone downloading the trojan and trying to run it is asked to install a codec that will play the supposed media.

McAfee said such a large outbreak was rare because hi-tech criminals typically prefer to target their malicious creations to keep numbers manageable and to avoid detection.

In the last seven days the trojan had been found on more than 500,000 of the PCs that notify the company when a malicious file is downloaded. So far, only 10% seem to have gone as far as to install the fake codec and be plagued with pop-ups.

Other security companies have not seen the trojan in such large numbers as McAfee. Only those using Windows are vulnerable to the malicious program.

McAfee has urged users to update their security software and be wary when using file-sharing networks.