Dubbed the Chameleon botnet, Spider.io said it is notable for the size of its financial impact. If the numbers hold true, the Chameleon botnet is costing display advertisers at least 70 times more than the Bamital botnet that Microsoft and Symantec took down in February.

But the security firm said the Chameleon botnet is arguably even more notable for the fact that it's the first found to impact display advertisers at scale , as opposed to text-link advertisers. Spider.io reports more than 120,000 host machines have been infected, and 95 percent of those machines access the Web from residential IP addresses in the United States.

"Display advertisers use algorithms with varying degrees of complexity to target their advertising at the most appropriate Web site visitors," Spider.io said in a blog post. "These algorithms involve continually measuring Web sites and their visitors to determine engagement levels with Web site content and with ad creatives. For the Chameleon botnet to evade detection and to impact display advertisers to the extent that it has requires a surprising level of sophistication."

A True Chameleon

Graham Cluley, a senior security consultant at Sophos, told us that despite the work among search engine giants to curb it, click fraud is still an issue.

"The Chameleon botnet, and other attacks like it, are mimicking the clicks made by Internet users to make it appear as though online advertising campaigns are working, driving traffic to advertisers' Web sites," Cluley said.

In Chameleon's case, he explained, it makes the click look more "human" by randomly moving the cursor and the place where the mouse clicks, and pretending to be Internet Explorer 9.0 running on Windows 7. However, he added, if advertising networks were to distrust clicks from IE9 running on Windows 7, that would obviously block a large number of legitimate clicks.

Danger to Consumers?

"Advertising networks -- not the advertisers themselves -- need to work harder at identifying the difference between a genuine user clicking on an ad, and a compromised computer that has been turned into a click-fraud bot," Cluley said. "That's not necessarily an easy challenge to overcome."

But what about the end user? Is there any threat to consumers here? Does the botnet do anything else harmful to the victim's computer or just use them as a mule?

"There's nothing to stop a botnet being used in ways that would be financially harmful to the PC owner," Cluley said. "Obviously even if it only conducted click fraud, it would be eating up bandwidth and computer resources, but most likely it would be doing other 'bad stuff' too."

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