Phony Web Traffic Tricks Digital Ads

Authorities and Internet-security experts say tens of thousands of dubious websites are popping up across the Internet. Their phony Web traffic is often fueled by "botnets," zombie armies of hijacked PCs that are controlled from unknown locations around the world, according to Internet security experts.

The sites take advantage of the simple truth that advertisers pay to be seen. This creates an incentive for fraudsters to erect sites with phony traffic, collecting payments—often through middlemen and sometimes directly from advertisers. At their most sophisticated, botnets can mimic the behavior of online consumers, clicking from one site to the next, pausing at ads, watching videos, and even putting items in shopping carts.