Late afternoon on November 30, Mozilla rushed out an emergency update for its open-source Firefox web browser, fixing a zero-day vulnerability that was being actively exploited by attackers. The vulnerability was used in attacks against the Tor web browser which is based on Firefox.
The first public report of the Tor Browser attack emerged on November 29, in a post on the Tor mailing list. The Tor project is an effort that makes use of a global network of routers that aims to help provide a degree of anonymity and privacy for users. The TorBrowser is built on top of Firefox and integrates the Tor network, as well as additionally privacy tools.
Among the configuration options that are part of the Tor Browser is a feature called the security slider, which can reduce the potential attack surface of the browser. As the security slider level is increased from low to high, browser capabilities that have been known to have had vulnerabilities in the past are progressively disabled.
"If you slide your security slider to high, you won't be vulnerable to this issue," Roger Dingledine, co-founder of Tor wrote in a mailing list message.
The actual vulnerability in Firefox is identified as CVE-2016-9079 and is a remote code use-after-free memory flaw in the SVG animation library used in Firefox. The flaw could have been used by an attacker to effectively de-anonymize a user, providing the victimized system's IP and MAC address.
In a bugzilla code tracking entry that details the flaw, Daniel Veditz, security lead at Mozilla commented that an administrator at the obscuredfiles.com received a sample of an attack against the SVG vulnerability as an email attachment. The actual payload of the exploits works on Microsoft Windows systems, though the same underlying flaw in SVG also exists on macOS and Linux versions of Firefox as well.
The patch for CVE-2016-9079 is now present in the Firefox 50.0.2 update as well as the Tor Browser 6.0.7 release.
While it's not currently clear which group of attackers might have been exploiting the CVE-2016-9079 vulnerability, in a blog post, Veditz notes that the attack has similarity to one used by Federal Bureau of Investigation (FBI). In May, Mozilla launched a legal challenge to the FBI to disclose details about software flaws that might impact the Firefox Web browser. The FBI made use of an undisclosed technique in order to de-anonymyze TorBrowser users as part of a criminal investigation.
"If this exploit was in fact developed and deployed by a government agency, the fact that it has been published and can now be used by anyone to attack Firefox users is a clear demonstration of how supposedly limited government hacking can become a threat to the broader Web," Veditz wrote.
Sean Michael Kerner is a senior editor at eWEEK and InternetNews.com. Follow him on Twitter @TechJournalist