Update on 11/30/2016 2:20 PM California time: Firefox developer Mozilla and Tor have patched the underlying vulnerability, which is found not only in the Windows version of the browser, but also the versions of Mac OS X and Linux.
There’s a zero-day exploit in the wild that’s being used to execute malicious code on the computers of people using Tor and possibly other users of the Firefox browser, officials of the anonymity service confirmed Tuesday.
According to security researchers who analyzed the code, it exploits a memory corruption vulnerability that allows malicious code to be executed on computers running Windows. The malicious payload it delivers, according to an independent researcher who goes by the Twitter handle @TheWack0lian, is almost identical to one that was used in 2013 to deanonymize people visiting a Tor-shielded child pornography site. The FBI ultimately acknowledged responsibility for the exploit, which was embedded in webpages served by a service known as Freedom Hosting.
“It’s basically almost EXACTLY the same as the payload used in 2013,” TheWack0lian told. “It exploits some vuln that executes code very similar to that used in the 2013 Tor browser exploit. Most of the code is identical, just small parts have changed.”
Analysis of the 2013 attack is here. Where that attack sent a unique identifier to a server located at the IP address of 188.8.131.52, the new one sends data to a server at 184.108.40.206. The latter IP address is assigned to French Web host OVH. It wasn’t responding to queries at the time this post was being prepared.
This post will be updated in the coming hours in the event important new details become available.
Post updated in the 6th paragraph to correct the type of bug being exploited.