More than one million users of MySpace.com and other websites may have been infected with adware spread by a banner advertisement, 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, which is owned by VeriSign.
The advert exploits a problem in the way Microsoft's Internet Explorer browser handles WMF (Windows Metafile) image files.
The browser vulnerability raised alarms in December after hackers distributed a specially crafted WMF image through email, IM (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 sites 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 advert causes a download of a Trojan 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 causes infected machines to contact various 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.