The existence of the exploit, which is said to consist of one HTML and one CSS file, was first reported on the Tor Project mailing list. Tor Project co-founder and president Roger Dingledine said Mozilla is aware of the issue and is working on a patch. The Tor Browser, which is based on Firefox, will also be updated as soon as Mozilla releases the fix.
Enterprise security firm Trail of Bits has analyzed the exploit and determined that it leverages a use-after-free vulnerability affecting the SVG parser in Firefox. Others suggested it was a heap buffer overflow, but Trail of Bits said it was “fairly confident” in its analysis.
Trail of Bits CEO Dan Guido explained on Twitter that these types of flaws were discovered in WebKit years ago, but they still haven’t been addressed in Firefox. The vulnerability is not easy to exploit in Google Chrome and Microsoft Edge due to memory partitioning, a mitigation that is not present in Firefox, Guido said.
The vulnerability apparently also affects Mac OS, but the exploit seen in the wild is designed to target only Windows machines. It is unclear who created the exploit, but Trail of Bits believes it was written from scratch.
The researcher known online as “TheWack0lian” has analyzed the payload delivered by the exploit and determined that it’s similar to the one used by the FBI in 2013 to identify Tor users suspected of being child-pornography traders.
That was not the only time the FBI was believed to have used a Firefox zero-day vulnerability to unmask alleged criminals on the Tor network. Earlier this year, Mozilla asked a court in the Western District of Washington to require the government to disclose a flaw exploited by law enforcement in 2015 in an operation aimed at child pornography suspects.