Although all Tor users are potentially vulnerable it appears that the malware, which is exploiting the bug, targets only Windows users. The vulnerability allows arbitrary code execution and the observed attack appears to collect the hostname and MAC address of the Tor user and send them to a remote web server. According to the Tor project, “it’s reasonable to conclude that the attacker now has a list of vulnerable Tor users who visited those hidden services.”
While outlining what users can do, besides upgrade to the latest version of the Tor Browser Bundle which contains a fixed version of Firefox, the email suggested that, “switching away from Windows is probably a good security move for many reasons.”
The malware used to discover the identities of the Tor users is possibly linked to the FBI as on Friday a vast number of “hidden services” disappeared from Tor and a man from Ireland was arrested on a warrant issued by the FBI in connection with child porn charges which allegedly used the Tor network.
According to the Electronic Frontier Foundation, which issued a statement about the attack, the Tor anonymity tool is often used by human rights activists, journalists, political dissidents and whistleblowers since it allows them to use the web anonymously and avoid different surveillance and censorship techniques.