SAN FRANCISCO—A state appellate court in California has revived a defamation lawsuit that software company ZL Technologies Inc. filed against the authors of critical posts on the anonymous job review website Glassdoor.com.

The ruling overturns a finding from the trial judge in Marin County Superior Court who declined to force Glassdoor Inc. to hand over the names of the anonymous posters after finding the reviews contained opinion statements protected under the First Amendment.

On Thursday, the First Appellate District found six of the seven reviews in question contained factual content that could be the subject of a defamation claim if proven false.

The decision on the whole is a victory for ZL Technologies of Milpitas, California, which provides electronic content archiving software used in eDiscovery, compliance and analytics.

But Justice Maria Rivera wrote the company and its lawyers must first prove the statements are false before it can enforce a subpoena asking Glassdoor to unmask the commenters at the trial court below.

Rivera wrote “the constitutional protections weigh in favor of requiring the plaintiff to make a prima facie evidentiary showing of the elements of defamation, including falsity, before disclosure of a defendant’s identity can be compelled.”

ZL Technology’s lawyer, Michael von Loewenfeldt at Kerr & Wagstaffe, said Monday that proving the posts include false statements will be an “extremely low hurdle.”

“The burden on plaintiffs is not knowing what the rules are” in these cases, said von Loewenfeldt. “With a clear standard, we’ve got no difficulty meeting that standard,” he said.

Twitter Inc. and Public Citizen weighed in with amicus briefs at the Court of Appeal, largely backing Glassdoor’s position that strong First Amendment and privacy protections should apply to user anonymity in online speech. But von Loewenfeldt said that, in this instance, Glassdoor is attempting to shield content that goes well beyond protectable opinion, parody or political speech. “If a person says ‘This restaurant serves a rat and calls it chicken,’ that’s either true or not true,” von Loewenfeldt said. “Once you get to defamation, you’re no longer protected by the First Amendment.”

In a statement issued through an outside spokesman, Glassdoor said the company is “committed to protecting the First Amendment rights of our users to post anonymous reviews about their workplace experiences and opinions without fear of retaliation.”

“In this case, we are pleased the court has broadened evidence requirements for plaintiffs seeking to unmask anonymous speakers,” the company said.