Can an adblocker violate a website's copyright?

Adblockers are an integral part of the Internet landscape, beloved by users but a thorn in the side of publishers dependent on ad revenue. The legal debate on this has traditionally been conducted in the area of competition law. A ruling issued on 31 July 2025 by the Bundesgerichtshof (Germany's highest court in civil cases) leaves open the possibility that ad blockers may constitute an infringement of the website owner's copyright. The case was not finally decided, but sent back to the appeals court to conduct a more thorough technical and legal investigation.

The facts and legal context

The case was brought by a major German publisher against the developers of the popular browser plugin AdBlock Plus. When a user visits one of the publisher's news websites, a package of files (such as HTML, CSS and JavaScript) is sent from the server to the user's browser. These files contain the instructions the browser needs to build and display the Web page, including the advertisements.

The publisher argued that this body of code and instructions should be considered a computer program protected by copyright under German copyright law (§ 69a UrhG). According to the publisher, the adblocker interferes with this program by:

1. Block retrieval of ad content from servers (Variant 1).
2. Hide elements recognized as ads so that they do not appear on the screen even though they are loaded (Variant 2, "Element Hiding").

The publisher argued that these interventions amounted to an unauthorized modification or editing of its computer program, which is an exclusive right of the author (§ 69c Nr. 2 UrhG).

The decision of the Federal Court of Appeals

The Hamburg Court of Appeal had previously rejected the publisher's claim. It reasoned that even if the Web site code were a computer program, the adblocker does not affect the program. The original files sent by the publisher remain unchanged; the adblocker affects only the execution of the program and the temporary data structures created by the browser, such as the DOM tree.

The Bundesgerichtshof overturned this decision. The highest court ruled that the appeals court had skipped a crucial step. One cannot judge whether there is infringement if one does not first accurately define exactly what the protected work is. Is the protected "computer program" just the initial HTML file? Or does the protection also include the dynamic structures generated by the browser based on it, such as the DOM tree and the CSSOM, which determine the final display?

Because this fundamental question remained unanswered, the conclusion that there was no program violation could not stand. The case was therefore referred back to the court of appeal for a new, more thorough analysis.

Legal analysis and interpretation: the crucial role of European law

The Bundesgerichtshof gave the court of appeal an important instruction: take into account the ruling of the Court of Justice of the European Union in the case "Action Replay" (C-159/23). This case was about software that affected the operation of PlayStation games to give players benefits such as unlimited "turbo boost."

In that ruling, the Court of Justice clarified that the copyright protection of software (under the European Software Directive 2009/24/EC) extends to the mode of expression of the program (the source and object code), as it enables its reproduction. However, the protection does not extend to the content of variable data that a program places in the computer's working memory during execution. Thus, changing the value of a variable (e.g., setting the turbometer from 100% to infinity) is not a change to the protected computer program itself.

Want to know more about this ruling? 👉 Read Mr. Joris Deene's annotation in Auteurs & Media

The parallel with the adblocker case is obvious. The adblocker's defense will be that it only affects display by modifying temporary values or properties in the browser (e.g., setting the CSS property "visibility" to "hidden"), which is similar to changing data in working memory. The publisher, on the other hand, will have to show that the intervention goes deeper and changes the executable code itself - the instructions to the browser.

This case law is also relevant to Belgium where computer programs are also protected by copyright (art. XI.295 Code of Economic Law) and modifications thereof are not permitted (Art. XI.298).

What this specifically means

• For website publishers: The door is ajar to challenge adblockers through copyright law. However, this requires a complex technical burden of proof: they must show that their Web site code is an original, protected "computer program" AND that the adblocker makes an unauthorized change to the code, not just the data during execution.
• For adblocker developers: While there is no direct ban, the legal risk is increasing. The Court of Justice's "Action Replay" ruling provides a strong line of defense, but the outcome of the German proceedings could set an important precedent for all of Europe.
• For Internet users: Fundamentally, this debate touches on the control users have over their own browsers. If customizing the display of a Web site is considered a copyright infringement, it could have far-reaching implications for numerous browser extensions and the user's digital autonomy.

Frequently asked questions (FAQ)

Is using an adblocker now illegal in Germany?
No, it does not. This ruling only obliges the Hamburg Court of Appeal to re-examine the claim of copyright infringement in more detail. The final outcome has not yet been determined.

What exactly is the difference with the European "Action Replay" case?
The European Court ruled that modifying variable data in working memory (e.g., the number of lives in a game) is not an infringement of software copyright. The key question now to be answered in Germany is whether an adblocker only modifies such data (e.g., a display setting) or whether it modifies the underlying program code itself (the instructions executed by the browser).

Why is a Web site considered a "computer program"?
Modern Web sites are more than static text documents. They contain executable scripts (such as JavaScript) that give the browser dynamic instructions on how to build the page, what data to retrieve, and how to interact with the user. This functional and instructional nature makes the code legally qualify as a computer program.

Conclusion

The Bundesgerichtshof ruling marks an important shift in the debate over adblockers. The focus shifts from competition rules to the fundamental principles of copyright. The final decision will depend on a highly technical analysis of the interaction between Web site code and browser plugins. It is a battle that will help determine the future of online publishing and user freedom.