A court order has prohibited three students at the Massachusetts Institute of Technology who discovered a potentially costly flaw with subway fare cards from presenting their research at a hacker convention this weekend, and the three are facing a lawsuit filed by the Boston transit agency.

According to The Tech, a student newspaper at MIT, the students were surprised when the lawsuit was filed on Friday because they had already discussed their findings with the agency, the Massachusetts Bay Transportation Authority. Their research, for a class project at MIT, demonstrated that the magnetic fare cards could be reprogrammed to show a balance of more than $600, allowing people to ride the subway free.

In a news release, an official of the Electronic Frontier Foundation, an advocacy group that is advising the students, called the court order “an illegal prior restraint on legitimate academic research.”

The student newspaper pointed out that much of the information the order sought to suppress was already available online, in part through the transit agency’s court filing. Copies of the court order and other documents in the case have been posted on the newspaper’s Web site.

In a similar case in the Netherlands this year, a chip manufacturer sued researchers who found that transit-system cards could be cloned, but a court ruled in the researchers’ favor, allowing them to proceed with plans to publish their results. —Charles Huckabee