The three Massachusetts Institute of Technology students who were barred by a court order from discussing subway card vulnerabilities are now free to say what they want.

In a ruling likely to be cheered by computer researchers, a US federal judge let the 10-day-old gag order expire. US District Judge George O'Toole Jr refused to grant a preliminary injunction requested by the Massachusetts Bay Transportation Authority that would have blocked the students from talking about their findings until January 1, 2009.

The MBTA's requested injunction would have replaced a temporary restraining order granted during the Defcon hacker conference, which under federal court rules automatically expires on Tuesday in the US.

The MIT students planned to make a presentation at Defcon on security vulnerabilities in the Massachusetts transit authorities electronic card and ticketing system. But a different federal judge who was on duty that weekend blocked the presentation after MBTA sued the students and MIT.

Judge O'Toole said he disagreed with the basic premise of the MBTA's argument: that the students' presentation was a likely violation of the Computer Fraud and Abuse Act, a 1986 federal law meant to protect computers from malicious attacks such as worms and viruses.

Many had expected Tuesday's hearing to hinge on First Amendment issues and what amounts to responsible disclosure on the part of computer security researchers. Instead, O'Toole based his ruling on the narrow grounds of what constitutes a violation of the CFAA.

On that basis, he said MBTA lawyers failed to convince him on two points: The students' presentation was meant to be delivered to people, and was not a computer-to-computer "transmission." Second, the MBTA couldn't prove the students had caused at least US$5,000 damage to the transit system. Lawyers for the MBTA claimed Tuesday they had proof the students had violated the law, but stopped short of specifying what they did.

It's unclear what transit officials will do next. Lawyers for the MBTA weren't immediately available after the ruling, but they could appeal O'Toole's ruling to the U.S. First Circuit Court of Appeals. Unless either side backs down or a settlement happens, a trial on the T's lawsuit against the students and MIT will eventually occur, but so far no date has been set.

Lawyers for the students, in a case that has generated more attention in local media concerned about problems in the transit system than it has among national media concerned about privacy issues, welcomed the judge's decision.

"This was a case of shooting the messenger," said Cindy Cohn, a lawyer with the Electronic Frontier Foundation, a San Francisco advocacy group that was representing the students along with the Massachusetts affiliate of the ACLU and the Fish & Richardson law firm.

But Ieuan Mahony, a lawyer for the Boston law firm Holland & Knight who is representing the MBTA, said the transit authority had no interest in chilling computer security research. Instead, he said it merely wanted to ensure a method for wide-scale fare violations wasn't disseminated.

Security researchers working for the MBTA spent the last several days working through a confidential 30-page analysis, which has not been made public, that students had sent to the court and T officials. The document detailed the complete method for breaking the local Charlie card payment system, including specific details the students say they didn't plan to reveal at the Defcon conference.

MBTA said in documents filed with the court said that fixing the security flaws would take five months. ("Students have the ability to cause significant harm to the CharlieTicket system, during the roughly five-month window that remedial actions will require.")

T officials concluded that the students had, in fact, found a way to break the paper Charlie card system, but had only found theoretical methods for breaking the plastic Charlie card, an RFID smart card that can have T fares electronic added to it.

Mahony said the 30-page analysis was a "very useful document," adding, it's "invaluable, but there are additional materials that cause us great concern." In particular, the transit authority wanted correspondence with Defcon officials and materials from their class with MIT professor Ron Rivest, a cryptographer best known as one of the co-inventors of the RSA public key encryption system, which is commonly used in e-commerce.

Despite the First Amendment implications of the case, O'Toole made it clear he intended to steer clear of the Bill of Rights. "I appreciate the breadth of views of others," he said, "but my views are considerably more limited." (Federal judges generally try to avoid constitutional issues if the dispute can be resolved by interpreting the text of a statute. In this case, it was a 1986 law that he decided didn't properly apply in this case.)

What the students intend to do now that the gag order has been lifted is unclear. If they wished, they could still make the Defcon presentation at some other forum. Cohn said she hasn't spoken with the three, who are still on summer break.

One of the students, Zack Anderson, told The Boston Globe in an interview published Monday that after the dust-up with the MBTA is done, he intends to work on a company that converts heat from a car's shock absorbers into energy for the car's engine. He reiterated in the interview that the students never intended to cause harm to the transit system.

"It wasn't to enable others to get a free fare or cause any sort of havoc," Anderson told the Globe. "It was really to show how major the issues are in this system, which also might resonate in many other systems around the world."

But one thing is certain: they have no intention of revealing the 30-page document that contained the specific details that told someone how to break the Charlie card system.

CNET News.com's Declan McCullagh contributed to this report.