Click Forensics also warns of a new malware scheme targeting display banner ads. A "sophisticated" program increases impressions by rotating banner ads every 10 to 15 minutes.
"We are investigating the malware-driven attacks in more detail, but early evidence points to an impression inflation scheme," Paul Pellman, CEO of Click Forensics, said in a press release. "It's something we will examine more closely and report on later this year."
Outside of North America, the countries that produced the most fraudulent traffic were Japan, The Netherlands, Philippines, Sweden and France.
Click Forensics' data is based on advertising campaigns on search engines, comparison shopping engines, online publishers, and social networks from more than 300 ad networks.
For more on click fraud, check out these Search Engine Watch articles: