An anonymous reader writes: A Firefox zero-day being used in the wild to target Tor users is using code that is nearly identical to what the FBI used in 2013 to unmask Tor-users. A Tor browser user notified the Tor mailing list of the newly discovered exploit, posting the exploit code to the mailing list via a Sigaint darknet email address. A short time later, Roger Dingledine, co-founder of the Tor Project Team, confirmed that the Firefox team had been notified, had “found the bug” and were “working on a patch.” On Monday, Mozilla released a security update to close off a different critical vulnerability in Firefox. Dan Guido, CEO of TrailofBits, noted on Twitter, that “it’s a garden variety use-after-free, not a heap overflow” and it’s “not an advanced exploit.” He added that the vulnerability is also present on the Mac OS, “but the exploit does not include support for targeting any operating system but Windows.” Security researcher Joshua Yabut told Ars Technica that the exploit code is “100% effective for remote code execution on Windows systems.” “The shellcode used is almost exactly the shellcode of the 2013 one,” tweeted a security researcher going by TheWack0lian. He added, “When I first noticed the old shellcode was so similar, I had to double-check the dates to make sure I wasn't looking at a 3-year-old post.” He’s referring to the 2013 payload used by the FBI to deanonymize Tor-users visiting a child porn site. The attack allowed the FBI to tag Tor browser users who believed they were anonymous while visiting a “hidden” child porn site on Freedom Hosting; the exploit code forced the browser to send information such as MAC address, hostname and IP address to a third-party server with a public IP address; the feds could use that data to obtain users’ identities via their ISPs.