More than one million users of MySpace.com may have been infected with adware spread by a single ad, according to iDefense, a computer security group.

The advertisement, for a site called deckoutyourdeck.com, appeared in user profiles on MySpace, an online community with at least 70 million users, said Ken Dunham, director of the rapid response team at iDefense, a subsidairy of VeriSign.

The ad exploits a problem in the way Internet Explorer handles Windows Metafile (WMF) image files.

The browser vulnerability raised alarms in December after hackers distributed a specially crafted WMF image through e-mail, instant messaging links and websites. If the image was opened, it could allow a hacker to gain control over a victim's computer.

There are at least 600 websites that take advantage of the WMF vulnerability, Dunham said. Microsoft issued a patch for the problem in January, but many consumer computers may not have applied the patch, leaving them unprotected.

Unpatched machines are particularly vulnerable. Merely visiting a page with the deckoutyourdeck.com banner ad causes a download of a Trojan horse program. Those who have installed the patch see a prompt asking to download a file called "exp.wmf" when visiting a page with the advertisement, Dunham said.

Once it starts to run, the Trojan in the banner ad causes infected machines to contact multiple websites and download, among other unwanted programs, advertising software from PurityScan. The PurityScan software can cause unwanted pop-up windows to appear, and also tracks a user's online activity.

Adware can be very difficult to remove, even for technically savvy users.

"The problem is hackers are using a variety of exploits -- especially WMF -- to illegally and silently install this [adware] on users' computers," Dunham said.

MySpace has increasingly been targeted by hackers because of its popularity. MySpace officials contacted in London Thursday afternoon had no immediate comment. iDefense's Dunham was not sure whether the banner advertisement has been taken down yet, but said that it could have been active for weeks.

Web sites that distribute adware are paid based on the number of machines that get infected with the software, and hackers have created ways to spread the adware without user consent, increasing their payments.

iDefense estimated the number of infections caused by the deckoutyourdeck.com ad through a server in Turkey hosting the adware. The server appears to track the number of machines infected with the adware, and indicated that 1.07 million computers had downloaded the program, Dunham said.

A Whois search for deckoutyourdeck.com leads to a winding trail of registrants. Dunham said hackers frequently use false credentials when registering a domain name to cloud inquiries.