A Javascript zero-day exploit is being actively exploited in the wild to unmask Tor Browser users. The vulnerability is believed to affect multiple Windows versions of the open source Firefox web browser. The exploit submits users’ machine details to a remote server hosted on a virtual machine in France. Mozilla is scrambling to patch the critical vulnerability, but it is still unknown who is behind the current exploit. It is similar to a separate Tor Browser exploit that emerged in 2013 targeting Tor users who accessed child pornography.
Source: https://thehackernews.com/2016/11/firefox-tor-exploit.html