An attack that exploits a Firefox vulnerability in JavaScript has been observed in the wild. Windows users using the Tor Browser Bundle (which includes Firefox plus privacy patches) appear to have been targeted. The attack appears to collect the hostname and MAC address of the victim computer, send that to a remote webserver over a non-Tor connection, and then crash or exit. It appears that TBB users on Linux and OS X, as well as users of LiveCD systems like Tails, were not exploited by this attack.”]
Source: https://lists.torproject.org/pipermail/tor-announce/2013-August/000089.html