Mozilla and Tor have published browser updates to patch a critical Firefox vulnerability used to deanonymize users (via ArsTechnica).

Privacy tool Tor is based on the open-source Firefox browser developed by Mozilla, which received a copy of the previously unknown JavaScript-based attack code yesterday. Mozilla said in a blog post that the vulnerability had been fixed in a just-released version of Firefox for mainstream users.



The code execution flaw was reportedly already being exploited in the wild on Windows systems, but in an advisory published later on Wednesday, Tor officials warned that Mac users were vulnerable to the same hack.



"Even though there is currently, to the best of our knowledge, no similar exploit for OS X or Linux users available, the underlying bug affects those platforms as well. Thus we strongly recommend that all users apply the update to their Tor Browser immediately."

The exploit is capable of sending the user's IP and MAC address to an attacker-controlled server, and resembles "network investigative techniques" previously used by law-enforcement agencies to unmask Tor users, leading some in the developer community to speculate that the new exploit was developed by the FBI or another government agency and was somehow leaked. Mozilla security official Daniel Veditz stopped short of pointing the finger at the authorities, but underlined the perceived risks involved in attempts to sabotage online privacy.



"If this exploit was in fact developed and deployed by a government agency, the fact that it has been published and can now be used by anyone to attack Firefox users is a clear demonstration of how supposedly limited government hacking can become a threat to the broader Web."

The Firefox attack code first circulated on Tuesday on a Tor discussion list and was quickly confirmed as a zero-day exploit – the term given to vulnerabilities that are actively used in the wild before the developer has a patch in place.

The latest Tor update that fixes the vulnerability is version 6.0.7 and can be downloaded here.

Vanilla Firefox users can download the update to their browser manually from here.