Google has identified and disabled 192 Chrome browser extensions that injected rogue ads into Web pages opened by users without being upfront about it. The company will scan for similar policy violations in future.

The action followed a study that the company conducted together with researchers from University of California Berkeley and which found that more than five percent of Web users who accessed Google websites had an “ad injector” installed.

The deceptive Chrome extensions were detected as part of that study, but the researchers also found ad injectors affecting browsers such as Internet Explorer and Mozilla Firefox, on both Windows and Mac OS X.

Google doesn’t specifically ban extensions published on the Chrome Web Store from injecting ads into Web traffic, as long as they clearly inform users about what they do, but the study found that around a third of extensions with such functionality were actually malware.

The larger problem is that rogue ad injection, which is used for advertising fraud, is not done only through browser extensions. There are software applications that can do this at the network layer, outside of the browser, or by hooking the browser process without installing an extension.

Some of these applications are outright malware, while others fall into a category that antivirus firms call “potentially unwanted programs,” or PUPs. Such was the case of Superfish, a program that Lenovo pre-installed on some of its consumer laptops. Unfortunately, aside from injecting ads into Web pages, the application also opened a serious security hole on users’ computers.

Google has recently taken action against PUPs as well, by starting to display warnings in Chrome whenever users try to download potentially harmful software. While commendable, the company’s effort will likely only stop a small percentage of PUP installations, because many programs of this type are not directly downloaded by users from the Internet. Instead, they are bundled with and installed by other freeware applications.

Moreover, ad hijacking programs that fall in the malware category are distributed through exploits, malicious email attachments and other nefarious methods that don’t involve Web downloads through the browser.