The analysis of over 100 million page views found that ad injectors were installed on both Windows PCs and Macs and that they had infiltrated the Chrome, Firefox and Internet Explorer web browsers. What's more, of those computers running an ad injector, 50 percent were running two, and nearly one-third had four or more.

An ad injector is an extension that comes to life when a PC is logged on to the web. It replaces the advertising that would usually be displayed on a webpage in the browser with different ads that the ad injector's creator wants you to see instead. Depending on what that ad is, the extension can be benign or malicious.

Their use has received a healthy dose of attention in recent weeks thanks to the outcry surrounding Superfish, an ad injector that Lenovo preinstalled on a number of its PCs.

As Google software engineer Nav Jagpal, puts it: "Injectors are yet another symptom of 'unwanted software' -- programs that are deceptive, difficult to remove, secretly bundled with other downloads, and have other bad qualities."

For instance, rather than serving up ads, they could serve up malware. Over one third of the ad injectors running on the Chrome browser examined as part of the study were identified as outright malware.