YouTube’s war on ad blockers is facing a legal challenge in the EU over Google’s use of JavaScript to determine when an ad blocker is in use.

YouTube has been increasingly warning users that they are not allowed to use ad blockers when visiting the site, even threatening to restrict playback if they do. The platform relies on JavaScript to determine the presence of an ad blocker, something that may be against the EU’s privacy laws.

According to The Register, privacy advocate Alexander Hanff has lodged a complaint with the Irish Data Protection Commission (DPC). The complaint alleges that YouTube’s use of JavaScript violates the law since Google does not first ask the user if they agree to their browser being profiled.

“In early 2016 I wrote to the European Commission requesting a formal legal clarification over the application of Article 5(3) of the ePrivacy Directive (2002/58/EC) and whether or not consent would be required for all access to or storage of information on an end user’s device which was not strictly necessary,” Hanff told The Register.

“Specifically whether the deployment of scripts or other technologies to detect an ad blocker would require consent (as it is not strictly necessary for the provision of the requested service and is purely for the interests of the publisher). The European Commission sent me a formal written response agreeing with my position that such activities would require consent.”

Hanff says the DPC agreed that YouTube’s actions could be a violation.

“In a call I had with the Irish DPC at the end of last week (after filing my complaint against YouTube), they did not disagree with my analysis and agreed to reach out to YouTube (Alphabet),” Hanff said. “I have since received another update that they have reached out to YouTube on Monday and will update me at the end of the week with any further information.”

If Hanff’s complaint is successful, it could significantly hinder YouTube’s efforts to combat ad blockers, at least in the EU.