Privacy advocate Alexander Hanff knew he had touched a nerve when the angry tweets from publishers and ad tech execs started pouring in.

Over the past few months, Hanff, CEO of the London-based privacy advocacy and consulting firm Think Privacy, has made the rounds both on Twitter and in regulators’ offices across Europe to take a stand against something that few would assume is worthy of involvement from courts and regulators: the publisher tactic of using scripts to detect when their readers are using ad blockers.

Hanff has waged his campaign against the practice on multiple fronts. In May he sent a cease and desist letter to technology media company IDG UK, one of the many publishers he claims violates his right to privacy (and European laws) by scanning his machine for adblockers without his permission. He plans to expand his cease and desist campaign in the coming months to publishers in almost a dozen countries, including France, Estonia, The Netherlands, Poland, Germany, and Sweden.

Publishers that defy him (as IDG UK already has) risk being on the wrong end of both regulatory action and even civil suits in European courts, both of which Hanff plans to rope into his campaign if he has to. He also plans to expand his attacks to companies such as PageFair, which not only detect adblocking activity, but also show users ads anyway.

CONGRATULATIONS @IDGUK – You have the honour of being 1st test case against #adblock detection. #privacy #securityhttps://t.co/DU0BNzxzGm

— Alexander Hanff (@alexanderhanff) May 27, 2016

With his attacks, Hanff is on the offense against what was already a defensive move by publishers, most of which are still trying to figure out the best way to counter the mounting problem of adblocking. For the likes of publishers such as Forbes and Wired, the first step to solving adblocking is simply to determine how many of their readers are using adblockers in the first place. Only then can they start the dialogue with readers to inform them of the relationship between free content and digital advertising. The concern over adblocking is particularly dire in European countries such as Germany and Poland, where adblocking rates are as high as 38 percent.

Hanff, for his part, says he’s sympathetic to publishers struggles’ with adblocking, but argues that it’s their reliance on third-party advertisers and the ad ecosystem that’s the real issue. “The problem is ad blocking is being seen as a disease, when in reality it’s the symptom of one: the advertising on their pages.”

Perhaps predictably, Hanff’s campaign hasn’t won over many publishers. But it’s not only publishers that feel threatened by his campaign. Hanff has been subject to a barrage of angry emails and tweets from employees at ad-tech companies and creators of anti-adblocking tools who aren’t crazy about his tactics either. Hanff says he isn’t deterred by the backlash, but rather emboldened by it.

“When you’re working in this field, if you make people angry, then they’re clearly worried — and if they’re worried, then that means that you’re right,” he said. “The amount of ire and vitriol that has been thrown my way over the past four or five months is a very clear indication that not only am I right, but also that these people are absolutely terrified.”

In reality, the issue is far less clear-cut. Hanff’s campaign is based on his interpretation of article 5.3 of the European Union’s 2009 ePrivacy Directive, known as the “Cookie Law,” which says that readers must give consent before publishers can store or access information stored on their devices. Scanning a user’s system to see if they’re using an adblocker, in Hanff’s view, violates that provision.

Hanff isn’t alone in his assessment. The European Commission, which he contacted last year, wrote back in May to agree with Hanff’s reading of the law. “Article 5.3 does not limit itself to any particular type of information or technology, such as cookies,” the European Commission wrote. “In light of the above, Article 5(3) would also apply to the storage by websites of scripts in users’ terminal equipment to detect if users have installed or are using ad blockers.”

Some privacy experts said that Hanff’s claims are, at least on first glance, legitimate. “I think in a strict intrerpretation of the law, it does hold up,” said Geoff Revill, a privacy advocate and CEO of UK-based social network Krowdthink. Frederik Borgesius, a researcher at the University of Amsterdam’s IViR Institute for Information Law, said that “detecting whether a website visitor has an adblocker by storing a script on that visitor’s computer seems to require the visitor’s prior consent.” But he stopped short of saying whether Hanff had a slam dunk case.

Likewise, there is skepticism from industry bodies such as IAB Europe, which has denied the claim that publishers are violating the law when they attempt to detect adblockers. Hanff’s case is “based on a very extreme interpretation” of the law’s broad language, said Townsend Feehan, CEO of IAB Europe. “Hanff jumps up and down and makes a lot of noise, but I don’t see a lot of nodding heads around Brussels or elsewhere that shows that there is wide uptake for his ideas.”

Others have pushed back as well. James Roven, the developer of BlockAdBlock, an anti-adblocking tool, wrote in a blog post in May that Hanff’s interpretation is based on a fundamental misunderstanding of how adblocker detection works, and a misreading of the ePrivacy directive.

“Making an educated guess based on browser behavior is absolutely not the same as accessing stored information. The conclusions reached by the EC are the result of a mischaracterization of anti-adblock defense technology — either intentional or otherwise,” Roven wrote. (Hanff, who has a degree in computer science and has been a privacy advocate for over a decade, maintains that he understands the technical aspects better than most.)

Skepticism aside, IAB Europe is playing it safe. Earlier this month, the group released a set of adblocker detection guidelines aimed at publishers looking to err on the side of caution. For more paranoid publishers, the group recommends the creation of either “consent banners” informing readers of the existence of adblocking detection scripts or a “consent wall,” which first-time visitors of the site would see before getting to the rest of a publisher’s site. The hope is that, by making it easy as possible for users to give consent, publishers will be avoid being on the wrong side of the law.

Hanff, who has been outspoken on all of this, was decidedly less forthcoming when I asked him to lay out an adblocking detection technique that wouldn’t land publishers in legal hot water. “There are ways to detect adblockers that aren’t illegal. I’m not going to go into what those are because I’m certainly not going to do publishers’ jobs for them,” he said. “If they want my advice on how to do it legally, they can pay me for it.”

In the end, however, Hanff says his goal isn’t to sue publishers, or even make money off of all of this. “I’m hoping it doesn’t get to the point where I have to take anyone to court. But if necessary, I will take this as far as I can take it.”