Mozilla is fighting to force the FBI to disclose details of a vulnerability in the Tor web browser. The company fears that the same vulnerability could affect Firefox, and wants to have a chance to patch it before details are made public.

The vulnerability was exploited by FBI agents to home in on a teacher who was accessing child pornography. Using a "network investigative technique", the FBI was able to identify the man from Vancouver, but Mozilla is concerned that it could also be used by bad actors.

Perhaps unsurprisingly, the government says that it should be under no obligation to disclose details of the vulnerability to Mozilla ahead of anyone else. But the company has filed a brief with a view to forcing the FBI's hand. The argument is that users should be kept protected from known flaws by allowing software companies to patch them.

Denelle Dixon-Thayer, Chief Legal and Business Officer at Mozilla Corporation, said:

The Tor Browser is partially based on our Firefox browser code. Some have speculated, including members of the defense team, that the vulnerability might exist in the portion of the Firefox browser code relied on by the Tor Browser.

She added:

At this point, no one (including us) outside the government knows what vulnerability was exploited and whether it resides in any of our code base. The judge in this case ordered the government to disclose the vulnerability to the defense team but not to any of the entities that could actually fix the vulnerability. We don’t believe that this makes sense because it doesn’t allow the vulnerability to be fixed before it is more widely disclosed.

Mozilla argues that disclosing the vulnerability to allow patching will help to make the web more secure, but the FBI is unlikely to want to give up what it will clearly see as a valuable weapon (a backdoor by another name, essentially) in its arsenal.