To carry out the operation:

The Methbot forgers first took numeric internet addresses they controlled and falsely registered them in the names of well-known internet service providers.

Among those were Comcast, AT&T and Cox, as well as fake companies like AmOL. This allowed the thieves to make it look as if the web traffic from Methbot’s servers in Dallas and Amsterdam were really coming from individual users of those internet providers.

The forgers then associated the addresses with 571,904 bots designed to mimic human web surfers.

Embedded in the bots’ web browsers were fake geographic locations, a fake history of other sites visited and fake logins to social networks like Facebook. “The bots would start and stop video just like people do and move the mouse and click,” Mr. Tiffany said.

The perpetrators connected the bots to the automated advertising networks that sell unsold ad space for thousands of websites.

A bot would pretend to visit a website like CNN.com, and the ad networks would conduct a microsecond bidding war against one another to show a brand’s video ad. But instead of going to the real CNN, the bot’s web browser would go to a fake site that nobody could see, and the ad would play there.

Finally, the system would report fake data to the ad networks and advertisers to convince them that humans had watched the ad on the real content site.

“It would send just the right kind of metrics back to look like real live audiences that were logged into Facebook and watching videos all day,” Mr. Tiffany said. The thieves then collected payment for the ads.

The report did not name the advertisers tricked by the fraud.

David Hahn, the executive vice president of strategy at Integral Ad Science, an advertising security firm that competes with White Ops, said the Methbot fraud affected just a tiny portion of the ad traffic of his own clients.

“There are new bots and new ways in which the bad guys are trying to figure out ways around our technology all the time,” he said.

The automated ad networks that buy and sell access to ad space on popular websites operate in a murky, fast-paced world, and it’s often unclear to advertisers who such middlemen truly represent.