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Mozilla Pushes for FBI Disclosure of Tor Browser Zero-day

The ongoing battle over the Federal Bureau of Investigation’s (FBI) use of a zero-day in the Tor anonymity browser hit a new gear this week with Mozilla filing a brief to get access to the  vulnerability details.

The ongoing battle over the Federal Bureau of Investigation’s (FBI) use of a zero-day in the Tor anonymity browser hit a new gear this week with Mozilla filing a brief to get access to the  vulnerability details.

The brief [PDF] filed with the U.S. District Court for the Western District of Washington, warns that “the security of millions of individuals using Mozilla’s Firefox Internet browser could be put at risk by a premature disclosure of this vulnerability.”

Tor, popular among web users for the privacy and anonymity features it offers, consists of a modified Mozilla Firefox web browser. 

The open-source Mozilla now wants to make sure its own code isn’t implicated in the Tor zero-day that was used by the FBI in 2015 to unmask web users accessing child pornography content.

In a blog post, Mozilla Chief Legal and Business Officer Denelle Dixon-Thayer writes:

The relevant issue in this case relates to a vulnerability allegedly exploited by the government in the Tor Browser. 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. 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.

“If our code is implicated in a security vulnerability, [the] government must disclose the vulnerability to us before it is disclosed to any other party. We aren’t taking sides in the case, but we are on the side of the hundreds of millions of users who could benefit from timely disclosure,” Dixon-Thayer added.

The Mozilla brief is urging the court to require the government to disclose the vulnerability to the affected technology companies first, so it can be patched quickly. 

During the criminal case proceedings, Justice Robert J. Bryan ruled that the FBI to reveal the code it used to track the defendants but the government refused, arguing that the details of the exploit was not necessary for the defense’s case.

Written By

Ryan Naraine is Editor-at-Large at SecurityWeek and host of the popular Security Conversations podcast series. He is a security community engagement expert who has built programs at major global brands, including Intel Corp., Bishop Fox and GReAT. Ryan is a founding-director of the Security Tinkerers non-profit, an advisor to early-stage entrepreneurs, and a regular speaker at security conferences around the world.

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