In November last year, some users of Yahoo’s mail service began to have trouble logging on to read their e-mail. These people were all using ad blocking software. Yahoo was hunting for this software and then preventing users from signing in until the ad blocker had been switched off.

The company said the exercise had been a small scale test involving only a few users in the U.S. But the implications were clear. Yahoo and other publishers on the Web are losing patience with ad-blocking software. They are preparing to fight it with a new generation of software that detects ad blockers.

An important question, then, is how the battle lines are being drawn up and how many websites are following Yahoo’s lead. Today, Muhammad Haris Mughees at Hong Kong University of Science and Technology and pals reveals the results of the first large-scale analysis of ad-block detection on the Web. And their conclusion is dramatic. “The clash between ad-blockers and ad-block detectors has resulted in a new arms race on the web,” they say.

The conflict arises because online advertising plays a crucial role in keeping online content free. But it also fuels profit margins for publishers and that’s why many attempt to maximize ad impressions using techniques such as auto play video ads, rollovers, and pop-ups along with increasingly aggressive tracking techniques to profile Web users.

These techniques significantly degrade the browsing experience for readers. As a result, many Web users are increasingly using ad blockers to prevent these annoying intrusions.

An increasing number of websites are using ad-block detection.

That’s caused a problem. In 2014, ad blockers reduced the income that online advertising generates by some $22 billion, a number that was almost double the losses from the year before.

So publishers are arming themselves with a weapon that fights back—ad-block detectors. These are scripts that sit on websites looking for the tell-tale signs that a browser has an ad blocker. It then asks the reader to switch it off and may even withhold content until they do.

The questions that Mughees and co set out to answer: How widespread is this behavior among publishers? And what range of techniques do they use to combat ad blockers?

To find out, they designed a machine-learning algorithm that crawls the Web looking for ad-block detectors and set it loose on the Alexa top 100,000 websites. “Overall, we found 1,089 ad-block detecting websites in the Alexa top-100K list,” they say.

Most of these employ passive techniques for spotting and responding to ad blockers. “About 300 websites respond to ad-block detection by requesting users to disable ad-blockers,” say Mughees and co. Many of these are relatively straightforward for Web users to ignore or bypass.

But a few sites are beginning to go further by employing third-party ad-block detectors such as PageFair. These use more sophisticated techniques to display ads, even when ad blockers are present and are much harder to game.

“PageFair uses dynamic JavaScript and code obfuscation,” say the team. “For example, it involves assembling URLs on the fly to retrieve additional JavaScript codes.” This allows it to serve ads on the fly even when an ad blocker is present.

This is a clear example of the cat-and-mouse nature of the conflict. As one side changes its tactics, the other evolves a more sophisticated response, and so on.

The increasingly sophisticated nature of this arms race is no surprise given the stakes. In 2014, online advertising was worth $50 billion and was rising at a rate of more than 15 percent per year. That’s big bucks.

Interestingly, Google and Microsoft have taken a different approach to ad blockers. These companies have simply paid to have their ads whitelisted by the ad blockers. This cost is presumably born first by the advertisers and ultimately by the consumers who buy the products.

So this is a war that, one way or another, is going to hit Web users where it hurts most: in their wallets.

Ref: arxiv.org/abs/1605.05841: A First Look at Ad-block Detection—A New Arms Race on the Web