A US District Court judge has tossed out evidence gathered by the FBI from Tor users, because the Feds wouldn't reveal how exactly it exploits their browsers to unmask them.

Jay Michaud, a Vancouver school administration worker, was charged with viewing a hidden service called Playpen, which hosted child abuse material, on the Tor anonymizing network. Tor works by running connections between users and servers through a large web of nodes, thus masking people's true IP addresses on the internet.

Unknown to Michaud, at the time he's accused of viewing the material, the server was already under the control of the Feds. The FBI had seized the system in February 2015 and ran it for a few weeks, adding their own server-side software to exploit a vulnerability in the Firefox-based Tor Browser and get visitors' public IP addresses and MAC addresses. The details of this vulnerability and how it was exploited by the FBI aren't known.

Michaud was arrested in July 2015.

In March of this year, the FBI refused to provide details of its “network investigative technique” (NIT) to the court, leading Michaud's lawyers to ask for the case to be dismissed.

Mozilla had backed the defence in the case, on the basis that if the FBI wouldn't reveal its techniques, browsers like its Firefox software couldn't be patched against vulnerabilities.

US District Judge Robert Bryan didn't demand the release of the exploit, but decided that the defence lawyers had a right to see it, so they could confirm that the FBI didn't breach the terms of the warrant they used to gather the data. And thus, the whole thing should be thrown out before it gets too Kafkaesque.

In an order on Wednesday [PDF], Bryan dismisses the evidence, writing: “For the reasons stated orally on the record, evidence of the N.I.T., the search warrant issued based on the N.I.T., and the fruits of that warrant should be excluded and should not be offered in evidence at trial. The court should not now order dismissal.”

The FBI's stance in this case is in contrast to the White House April 2014 assertion that government agencies aren't hoarding bugs. ®

Bootnote

Either inadvertently or deliberately, the court has also posted the deposition given by a security analyst working for the public defender in the case.

The filing, by Vlad Tsyrklevich, explains what the FBI refused to provide the defence.

Tsyrklevich was able to determine that the FBI worked out how to get IP address and another identifier (which might have been MAC address) from a target's machine.

However, that one component of the payload that the feds offered the defence didn't include how the payload was generated, what exploit the FBI used, or how their server collected data.

Tsyrklevich, currently identified as a security engineer for Square, says without the code for the client, he can't verify whether the FBI could be certain that it had a unique identifier for Michaud.

He adds that the server-side code is also vital evidence, since it would verify whether the FBI was storing the data it received properly. He cited the 2013 “watering hole” attack against Freedom Hosting, which served an NIT to people who were visiting legitimate sites as well as those surfing illegal content. ®