Facebook, X, YouTube and other social media platforms rely on a 1996 law to insulate themselves from legal liability for user posts. The protection from this law, Section 230 of the Communications Decency Act, is so significant that it has allowed tech companies to flourish.

But what if the same law could be used to rein in the power of those social media giants?

That idea is at the heart of a lawsuit filed in May against Meta, the owner of Facebook, Instagram and WhatsApp. The plaintiff in the suit has asked a federal court to declare that a little-used part of Section 230 makes it permissible for him to release his own software that lets users automatically unfollow everyone on Facebook.

The lawsuit, filed by Ethan Zuckerman, a public policy professor at the University of Massachusetts Amherst, is the first to use Section 230 against a tech giant in this way, his lawyers said. It is an unusual legal maneuver that could turn a law that typically protects companies like Meta on its head. And if Mr. Zuckerman succeeds, it could mean more power for consumers to control what they see online.

“I see and appreciate the elegance of trying to use a piece of law that has made user generated content possible, to now give users more control over those experiences and services,” he said.

Section 230, introduced in the internet’s early days, protects companies from liability related to posts made by users on their sites, making it nearly impossible to sue tech companies over defamatory speech or extremist content.

Mr. Zuckerman has focused on a part of Section 230 that spells out protection for blocking objectionable material online. In 2021, after a developer released software to purge users’ Facebook feeds of everyone they follow, Facebook threatened to shut it down. But Section 230 says it is possible to restrict access to obscene, excessively violent and other problematic content. The language shields companies from liability if they censor disturbing content, but lawyers now say it could also be used to justify scrubbing any content users don’t want to see.

Meta said it had asked the U.S. District Court for the Northern District of California, where the lawsuit was filed, to dismiss the case because Mr. Zuckerman had not released a software tool to clean up people’s Facebook pages. It also argued that Mr. Zuckerman had not shown that Section 230 should apply in his case.

“This suit is baseless, and was filed by the plaintiff over a hypothetical browser extension that he has not even built,” a company spokesman said.

The suit originates from a British software developer’s idea to reduce time spent on Facebook. In 2020, Louis Barclay, 35, a former Goldman Sachs analyst, manually unfollowed every person and group that filled his News Feed.

“I found it really revelatory that, almost overnight, I suddenly was not spending anywhere near the amount of time that I was spending before on Facebook,” he said.

So Mr. Barclay built a browser extension tool the same year that would automate the process, called Unfollow Everything. Roughly 12,000 people tried it, he said.

But on July 1, 2021, a law firm representing Facebook sent Mr. Barclay a cease and desist letter. His browser extension violated Facebook’s terms of service, including for “impairing the intended operation of Facebook,” the letter said. It also instructed Mr. Barclay to take down his browser extension or face a potential lawsuit.

Mr. Barclay complied. He also posted the letter to his website and wrote an opinion piece for Slate in October 2021.

Platforms “shouldn’t be able to wield the threat of lawsuits and account suspensions against researchers and developers who create tools that merely empower users,” he wrote.

That’s when Mr. Zuckerman, 51, decided to get involved. He had spent time in the tech world at a start-up and was credited with writing the code for the first pop-up ad. He later entered academia — taking over a research center at the Massachusetts Institute of Technology in 2011 — and wrote about how he was disenchanted with advertising technology.

Lawyers at the Knight First Amendment Institute at Columbia University were already discussing Mr. Barclay’s case and whether to pursue legal action. Mr. Zuckerman, who had served as a fellow at the Knight Institute during the early days of the coronavirus pandemic, connected with the lawyers there and agreed to be the face of any case the lawyers brought.

But he and his lawyers were still looking for a legal argument on which to hang their lawsuit. Preparing for a graduate-level class called “Fixing Social Media” in 2022, Mr. Zuckerman read Section 230 and noticed the provision protecting “technical means” to block objectionable content.

“I did have that sort of lightning bolt,” he said. He made a note and said to himself, “I think that means that we have a case here.”

Ramya Krishnan, one of his lawyers at the Knight Institute, agreed with Mr. Zuckerman, having combed through Section 230 herself.

“It just kind of just jumped out from nowhere,” she said.

Kaspersky, a Russian cybersecurity company, successfully used the same clause in Section 230 to defeat a lawsuit in 2007 brought by a company that made products Kaspersky categorized as malicious software. The cybersecurity firm argued that it was blocking objectionable content through protecting customers from the harmful malware.

Mr. Zuckerman is taking that argument a step further, asking the court to preemptively protect an effort to build software that filters content because an internet user simply does not want to see it.

“The purpose of the tool is to allow users who find the newsfeed objectionable, or who find the specific sequencing of posts within their newsfeed objectionable, to effectively turn off the feed,” Mr. Zuckerman’s lawyers said in the lawsuit.

The Knight Institute is pursuing the lawsuit in part because its lawyers believe it could provide a new path in a fractious debate over online speech, Ms. Krishnan said.

“It is one of these solutions on the table that I think does help us avoid both of these dystopias: on the one hand, a small coterie of corporations having excessive control over the digital public sphere, and on the other the government having the sweeping authority to set the terms and rules of this environment,” she said

If his lawsuit succeeds, Mr. Zuckerman plans to reproduce Mr. Barclay’s tool with an added component. People who use it will have the option of participating in a study Mr. Zuckerman is conducting that aims to measure whether users want more control over their social media.

“We’re pretty sure that this is something that we are allowed to do, and that people may be scared off from doing it because these are big companies with very good law firms,” he said. “But that feels like exactly the reason to be asking this question and asking a court to give us a pre-emptive ruling.”