In a victory for anonymous critics of corporations, a federal judge in San Francisco has rejected a company's attempt to force Yahoo to identify the online commenter who called the firm's chief executive a "known liar" who believes "humanity exists to be fleeced."

"The First Amendment protects the rights of individuals to speak anonymously," U.S. District Judge Susan Illston said in ruling against USA Technologies. She said a target of anonymous online attacks must have evidence that the postings violated its rights and caused serious harm before enforcing a subpoena to disclose the speaker's identity.

USA Technologies filed a suit in its home state of Pennsylvania in August 2009 against the poster known as Stokklerk and sent a subpoena to Sunnyvale's Yahoo, which hosts a message board about the company, demanding the person's Web address. The suit claimed defamation and securities fraud, alleging that Stokklerk had financial interests in denigrating the company.

Stokklerk went to federal court in San Francisco, Yahoo's home district, to quash the subpoena. Illston sided with the online critic, saying USA Technologies had offered no evidence of Stokklerk's financial involvement and could not prove defamation based on mere insults.

Stokklerk's comments last year about the company and its chief executive, George Jensen, were expressions of opinion rather than false statements of fact, the threshold for a libel case, Illston said in Monday's ruling.

The August 2009 posting that referred to Jensen as a "known liar" was "hyperbolic opinion," Illston said, because Stokklerk connected it to Jensen's assurances to investors that the company would be profitable that year. It was not.

USA Technologies also said it was libeled by Stokklerk's description of the company as a "soft Ponzi" and assertion that two top executives had "skimmed" more than $30 million from "the hugely unprofitable venture." But Illston said both postings were clearly meant as criticisms rather than as accusations of lawbreaking.

Attorney Matt Zimmerman of the Electronic Frontier Foundation in San Francisco, which represents Stokklerk, said suits such as USA Technologies' are a common tactic of companies trying to suppress criticism.

Lawyers for USA Technologies did not return messages seeking comment.