A recent appellate decision in a lawsuit involving cybersecurity company Malwarebytes could threaten ad-blocking tools, including the Electronic Frontier Foundation’s “Privacy Badger,” the digital rights group argues.

The decision, issued last month by the 9th Circuit Court of Appeals, allows Enigma to proceed with claims that Malwarebytes acted anti-competitively and engaged in false advertising, by flagging Enigma’s SpyHunter and RegHunter as potentially unwanted programs.

In its ruling, a panel of the 9th Circuit rejected Malwarebytes’ argument that it was immunized from suit by Section 230 of the Communications Decency Act. That law has a provision protecting computer services that offer tools to restrict objectionable material, but the appellate judges said the provision didn’t apply because Malwarebytes is a competitor to Enigma.

Malwarebytes recently asked the entire 9th Circuit to review that ruling.

The EFF is among a host of groups that are backing Malwarebytes’ request.

Among other arguments, the EFF says the decision threatens Privacy Badger, which allows users to prevent tracking by ad-tech companies.

“EFF’s ability to continue providing free privacy-enhancing tools to Internet users will be seriously threatened if the panel’s incorrect interpretation … stands,” the organization writes in a friend-of-the-court brief.

“The Enigma panel’s decision creates legal uncertainty for filtering tool providers that, in turn, promises to create a chilling effect to the detriment of Internet users,” the group writes. “Should the decision stand, filtering tool providers will seek to minimize their legal exposure by creating weaker, less effective filtering tools for fear of sweeping in competitors.”

A group of cybersecurity law professors also weighed in on Malwarebytes’ behalf. They argue that the panel’s ruling effectively paves the way for bogus lawsuits against security vendors.

“The panel decision will foster spurious legal accusations of anti-competitive blocking of software programs that are, in fact, dangerous to businesses and consumers,” they argue in a friend-of-the-court brief authored by Santa Clara University professor Eric Goldman and Seattle-based attorney Venkat Balasubramani. “These legal threats will hinder the ability of anti-threat software vendors to properly classify threats to businesses and consumers, which will make the Internet less safe for everyone.”

The Silicon Valley lobbying group Internet Association is also supporting Malwarebytes, arguing in a separate friend-of-the-court brief that the panel’s “misguided” ruling “opens the door to new attacks on self-regulatory tools that help make online communities safer and more accommodating.”