The Web is awash with “free” content—but of course we pay a price to view it: We have to negotiate a virtual minefield of ads, perhaps hundreds of them every day. These auto-play videos, rollovers and pop-ups can be a major nuisance, making content a challenge to read and discouraging some people from even trying.

Ad-blocking software has for years offered a respite, but not without collateral damage to companies placing ads no one sees. And now advertisers’ and ad blockers’ efforts to outmaneuver one another have come to a head; as blocking technology improves, some sites—publishers in particular—are penalizing viewers for using it. Firing back, developers are creating ad blockers they say are undetectable.

Blocking software hides advertising in a number of ways. Some kinds disable Web browser plug-ins—software add-ons you install to increase the types of content your browser can display—that deliver ads. Others identify and impede content from Web addresses known to serve ads. Digital ad spending reached a record $88 billion in the U.S. last year, according to the Interactive Advertising Bureau. But the amount wasted due to ad blockers is thought to be in the tens of billions—an obvious sore point with companies trying to maximize their exposure.

Many Web sites depend on advertising income, and sometimes deny access to content for people who install ad blockers. U.S. local news site, for example, in April began barring nonsubscribers from its pages if it detects ad blockers. Others have gone to more extreme lengths. In February lifestyle magazine Salon began asking online readers to help support the publication financially, either by shutting off ad blockers or allowing Salon to borrow users’ devices for cryptocurrency mining while they read. The site is testing a program that sends instructions to a reader’s processor to run calculations that allow Salon to earn a digital currency called monero.

Web sites that want to circumvent ad blocking must first be able to sense when a blocker is in use. Many of the most popular sites are getting better at that, according to a study presented in February at the Network and Distributed System Security Symposium in San Diego. Researchers at Syracuse University and The University of Iowa found just over 30 percent of the top 10,000 sites—as ranked by Web traffic analysis firm Alexa Internet—can detect ad blockers.

Stealth Mode

But countermeasures to such detection are becoming more common as well. Volunteer online communities are developing custom-made, ever-stealthier ad blockers or are making improvements to existing products. For example, users in one Reddit community discuss how to install special plug-ins and other relatively simple programs that make people’s ad blockers undetectable to sites they visit. One of the ways these systems work is by removing code designed to detect ad blockers from a Web page as it loads. Without the right kind of countermeasures in place, the site cannot tell that a visitor has installed such software in a browser. Another community-made ad blocker is Pi-hole, designed for use on devices built using Raspberry Pi—a small, single circuit board computer popular among hobbyists and tinkerers learning how to program. Like many ad blockers, Pi-hole has a long “filter” list of internet addresses from which ads are downloaded to the user. Pi-hole blocks those addresses at a network level, so none of the devices accessing the Web on that network—say, your home wi-fi—will download or even see those ads.

Arvind Narayanan, an assistant professor of computer science at Princeton University, last year got his students to help design a cunning blocker that could detect when Facebook ads had been designed to look like ordinary posts. Facebook had done this to evade traditional blocking techniques, which generally detect ads by looking for characteristic bits of code associated with advertising in a Web page. Narayanan’s system worked by taking a more holistic approach—it looked for words such as “sponsored” in a post, and flagged it as a possible ad (pdf).

Companies that sell ad blockers tend to be more upfront when pushing back against advertisers and publishers seeking to bypass their software. “It is kind of a cat-and-mouse game,” says Jutta Horstmann, a developer and head of filters at eyeo, maker of the popular Adblock Plus software. A difference between companies like eyeo and the community-based efforts is that Adblock Plus allows Web sites to detect its presence. Then the site can tell users it has spotted the ad blocker and can ask them to turn it off before allowing them to see the site’s content. Horstmann and her colleagues, however, designed Adblock Plus to prevent sites from serving ads to people who choose to keep the program running.

Like Pi-hole, Adblock Plus relies on large filter lists of internet addresses that it can block before they try to deliver ads to users. Eyeo’s software also has a list of known bits of code Web sites use to serve ads—and it can remove those specific bits when a page is loaded. “These filter lists are developed by an international open-source community, and they provide filters for a whole range of ad blockers,” Horstmann says. The firm currently updates its lists once a day and hopes to do so hourly in the near future, she adds.

Ad Blocker Wars

The escalation of tactics between advertisers and ad blockers reflects long-standing tensions over how to pay for content posted on the Web. The “ad blocker wars” were set up by the original Web model of giving away content for free, says Rita Gunther McGrath, a professor of management at Columbia University Business School. “It’s like saying, ‘Okay, we’re going to make money off the advertising printed on a milk carton and we’re going to give the milk away for free’,” McGrath says. She describes this as a “strategic mistake.”

One consequence has been advertisers increasingly relying on copious ad quantity and aggressive tactics to turn a profit, McGrath says. Rather than using ads strategically, today’s systems are a bit like an immature “teenager technology” that emphasizes volume over precision—which is why we are often served ads for things we have already purchased or that are only tangentially relevant to our interests, she says.

Some of those aggressive tactics, like Salon’s crypto-mining, might raise eyebrows—but they probably will not raise much revenue, and may be “a bit of a fad,” Narayanan says. Others say there is room for compromise. Earlier this year Google introduced a new ad blocker for its Chrome browser that instead of turning away all ads, filters out only those that violate standards created by the Coalition for Better Ads, a consortium of advertisers, content providers and others. The organization says it identifies ad experiences that fall beneath a “threshold of consumer acceptability.” Examples include pop-ups that take up too much screen space or try to catch people’s attention as they are leaving a particular site.


About 90 percent of Adblock Plus customers do not block all ads, according to Ben Williams, eyeo’s director of communications and operations. This statistic, he says, suggests those users will tolerate a certain amount of advertising. Eyeo recently launched an initiative to help compensate content creators who earn their money via ads. Dubbed Flattr, the system collects fees from users and distributes the money to Web sites or individuals (YouTube creators, example). The amount of payment depends on how much time users spend reading specific sites or watching certain videos. Another ad-blocking browser, Brave, has partnered with a group of music-related Web sites to offer them cryptocurrency payments from users. Even though ads are still blocked, the sites trying to make money with those ads still receive some compensation.

User frustration with ads is well documented and has played a major role in the back-and-forth between advertisers and ad blockers. Still, efforts to deny businesses the ability to profit from the content they publish have not solved the problem. Attempts to find some middle ground—allowing users the choice to compensate publishers and content creators instead of facing an avalanche of ads—may prove a good way to break the stalemate.