A number of Right Media ads have been infected with a Trojan virus, affecting dozens of websites on the Yahoo!-owned ad exchange .

News of the virus surfaced when ScanSafe reported that it saw an increase in a Trojan-Downloader.VBS.Agent it was blocking.

According to ScanSafe, up to 12 million infected ads were served across sites including MySpace, Bebo and Photobucket.

In a blog post, Right Media confirmed the virus, attributing the attack to an unnamed advertiser that inserted malicious creatives in the exchange. 

"We have always been, and continue to be fully committed to providing a safe marketplace for our members, their constituents and end-users," a Right Media statement read. "We've dedicated significant resources to Media Guard, Creative Tester and other protection mechanisms."

Right Media also said it plans to launch new safety measures in the future.

Dan Nadir, vice president for product strategy at ScanSafe, said the attack was a good example of how serious the malware problem is, pointing out that even trusted sites are vulnerable.

A ClickZ story reported that the ads required no user interaction, meaning that a simple page view could infect a user's computer with malware.