In a blog post, Google (GOOG) cited recent incidents involving ad injectors, warning that the injected ads harm user experience and compromise security. The company said it is responding to the problem by introducing a new filter for its DoubleClick Bid Manager service that will recognize the injectors and stop advertisers from bidding on them.

This is an important step not only for users, but for news publishers and other websites because the ad injectors typically overlay outside ads where the publishers own ads are supposed to appear. The Google blog post includes this extreme example, which shows an ad injector hijacking the New York Times:

Such ad injectors have been a problem since at least 2012, when one such operator robbed sites like CNN and the New York Times (GOOG) of millions of dollars by overlaying their own ads. The advertisers who purchase the ad slots from the injectors are not always aware they are doing so; in some cases, they may believe they are simply getting a good price for a spot on a popular site.

Some might wonder why users allow the ad injector software onto their computers in the first place. The answer is that they often don’t. In a recent controversy, the computer maker Lenovo was caught selling PC’s with pre-installed adware. And, in other cases, the ad injectors trick users into installing the software by secretly packaging it with free apps or software plug-ins.

The Google blog post also included a shot across the bow to the online “ad exchanges” where advertisers buy digital ads. This warning came in the form of a chart where Google showed that it can tell which ad exchanges are selling the ads on behalf of the shady installers. Google does not name names, but included this chart, which will no doubt set tongues wagging in the ad industry about which exchanges have all this compromised ad inventory:

As you can see there are four exchanges responsible for most of the problems, including one where over 15% of the ads are from injectors. Such exchanges may have an incentive to look the other way in the case of ad injectors since they receive a cut of the purchase price for all the ads sold.

Google’s announcement thus will come as relief to both publishers and honest advertisers since it is likely to significantly reduce the opportunities for ad injectors to ply their slippery craft. It will not, however, eliminate the practice since not every everyone uses Google DoubleClick Bid Manager, and it’s also possible some of the injected ads will slip through the new filter.

Google’s post also urged others in the industry to follow its example:

“We don’t expect the steps we’ve outlined above to solve the problem overnight, but we hope others across the industry take action to cut ad injectors out of advertising. With the tangle of different businesses involved—knowingly, or unknowingly—in the ad injector ecosystem, progress will only be made if we all work together. We strongly encourage all members of the ads ecosystem to review their policies and practices and take actions to tackle this issue.”

Here’s a recent video about the Lenovo scandal: