Spammers are using MP3 music files to get their messages past filtering software, according to a new report.

In its Intelligence Report for October 2007, Messagelabs claims that 15 million spam emails pretending to contain MP3 music files had been sent out to users.

The files were all named after well-known recording artists, with file names such as beatles.mp3, britney.mp3 and elvis.mp3.

However, instead of being serenaded with music, users who opened the files were subjected to a “rusty-sounding, 25-second voice-over” encouraging people to buy shares in a company.

Messagelabs said the size of the files was kept low, at around 50KB, to avoid detection from spam filters. It said the use of new technology showed how clever spammers were becoming.

Mark Sunner, chief security analyst for Messagelabs, said: “As users become wary of certain file attachments, scammers will move on to their next tactic, ever hopeful of finding the key which will easily open all inboxes rather than having the door slammed in their face by anti-spam filters.”

He predicted that with the rise in popularity of video sites such as Youtube, video spam and Powerpoint could be the next “format du jour”. The company also warned that spammers could apply the same approach to audio spam and upload the message to free multimedia hosting sites such as Google Video or Myspace.

Carole Theriault, a senior security consultant at Sophos, agreed with Messagelabs' findings, saying they had identified the same trend earlier this month, but did not believe MP3 emails alone would not be enough to infect a person’s computer.

“Spammers have become increasingly clever and are using new technology that will appeal to users. However, what is important to remember here is that the email itself will not lead to any viruses, it will be the link to a website or even a program download which may be incorporated into it,” said Theriault.

She advised people to be dubious of any emails sent by unknown companies and to report spammers to their ISP.

“People should not let their guard down because the email looks different and contains interesting things,” she added.

In its report, Messagelabs also found the number of infected websites had doubled in October, with 1,100 new infected sites being identified every day. Spam also increased by one per cent from September. However, the company also had some good news; it found that viruses spread by email were at their lowest level since April 2007.