A Javascript zero-day exploit for the TOR Browser which is based on Firefox Browser.
The critical vulnerability is believed to affect multiple Windows versions of the open source Firefox web browser as far back as Firefox version 41, and up to Firefox version 50. when exploit opened by a Firefox or Tor Browser with Javascript enabled on a Windows computer, it leverage a memory corruption vulnerability in the background to make direct calls to kernel32.dll, which allows malicious code to be executed on computers running Windows.
