Yahoo should refuse to run deceptive ads on its network, said Ben Edelman, an assistant professor at the Harvard Business School who studies Internet marketing practices. "It's hard to defend these ads' tactics. They intend to deceive, and by all indications they succeed," he said. "They have no proper place in Yahoo's ad network."

Yahoo bought Right Media for about US$700 million last year, looking to strengthen its position in its fight with Google for online advertising dollars. The network provides a marketplace for Web publishers who have been unable to fill all of their advertising spots, allowing unused ad inventory to be sold at auction to advertisers.

Government officials charged with enforcing deceptive advertising say that while the ads on Direct Media Exchange ads may be dubious, they may not be bad enough to warrant an enforcement action.

"Any type of advertisement that is likely to deceive consumers acting reasonably under the circumstances could be deceptive," said Rick Quaresima, an assistant director in the U.S. Federal Trade Commission's Division of Advertising Practices.

But the FTC looks at individual ads on a case-by-case basis, he said. And while ads that trick a consumer into visiting a Web site may be annoying, the ones that deceive in order to sell a bogus or harmful product such as malicious antispyware software are a top concern. "We're certainly more concerned with the cases of deceptive advertising that could cause tangible consumer harm or consumer injury. that would be where we would prioritize our enforcement efforts."