A critical denial-of-service (DoS) vulnerability was found in Facebook Fizz, the social media giant's open source implementation of the Transport Layer Security (TLS) protocol, Semmle reports.

If you're not familiar, Fizz enables secure communication with Web services and is used on most of Facebook's internal and external infrastructure. Facebook made the source code open source last August; since then, it was likely used by other organizations and projects. The bug was discovered by Kevin Backhouse of Semmle's research team and reported Feb. 20.

Semmle says the vulnerability is "relatively easy to trigger" for unauthenticated remote attackers. It's considered a DoS bug because it causes an infinite loop in Fizz and renders the service unavailable for other users. By exploiting this, an attacker could take down any infrastructure that relies on Fizz but couldn't gain unauthorized access to user data.

Facebook issued a patch (CVE-2019-3560) on Feb. 25, and its Web services are no longer vulnerable. Web applications dependent on Fizz are urged to upgrade their Fizz libraries. Semmle was awarded a $10,000 bug bounty for the discovery, which is out of the ordinary for DoS.

"While denial-of-service issues are typically not considered as part of our bug bounty program, this submission discussed scenarios which could have had significant risk," Facebook said in an email to Semmle. The security firm will donate the bounty to Techtonica (which doubles the donation to $20,000) and match the original $10,000 bounty with a donation to Community Servings.

Read more details here.

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