Earlier today, Cloudflare, along with Google and Amazon AWS, disclosed the existence of a novel zero-day vulnerability dubbed the “HTTP/2 Rapid Reset” attack. This attack exploits a weakness in the HTTP/2 protocol to generate enormous, hyper-volumetric Distributed Denial of Service (DDoS) attacks. Cloudflare has mitigated a barrage of these attacks in recent months, including an attack three times larger than any previous attack we’ve observed, which exceeded 201 million requests per second (rps). Since the end of August 2023, Cloudflare has mitigated more than 1,100 other attacks with over 10 million rps — and 184 attacks that were greater than our previous DDoS record of 71 million rps.

This zero-day provided threat actors with a critical new tool in their Swiss Army knife of vulnerabilities to exploit and attack their victims at a magnitude that has never been seen before. While at times complex and challenging to combat, these attacks allowed Cloudflare the opportunity to develop purpose-built technology to mitigate the effects of the zero-day vulnerability.

If you are using Cloudflare for HTTP DDoS mitigation, you are protected. And below, we’ve included more information on this vulnerability, and resources and recommendations on what you can do to secure yourselves.

Deconstructing the attack: What every CSO needs to know

In late August 2023, our team at Cloudflare noticed a new zero-day vulnerability, developed by an unknown threat actor, that exploits the standard HTTP/2 protocol — a fundamental protocol that is critical to how the Internet and all websites work. This novel zero-day vulnerability attack, dubbed Rapid Reset, leverages HTTP/2’s stream cancellation feature by sending a request and immediately canceling it over and over.  

By automating this trivial “request, cancel, request, cancel” pattern at scale, threat actors are able to create a denial of service and take down any server or application running the standard implementation of HTTP/2. Furthermore, one crucial thing to note about the record-breaking attack is that it involved a modestly-sized botnet, consisting of roughly 20,000 machines. Cloudflare regularly detects botnets that are orders of magnitude larger than this — comprising hundreds of thousands and even millions of machines. For a relatively small botnet to output such a large volume of requests, with the potential to incapacitate nearly any server or application supporting HTTP/2, underscores how menacing this vulnerability is for unprotected networks.

Threat actors used botnets in tandem with the HTTP/2 vulnerability to amplify requests at rates we have never seen before. As a result, our team at Cloudflare experienced some intermittent edge instability. While our systems were able to mitigate the overwhelming majority of incoming attacks, the volume overloaded some components in our network, impacting a small number of customers’ performance with intermittent 4xx and 5xx errors — all of which were quickly resolved.

Once we successfully mitigated these issues and halted potential attacks for all customers, our team immediately kicked off a responsible disclosure process. We entered into conversations with industry peers to see how we could work together to help move our mission forward and safeguard the large percentage of the Internet that relies on our network prior to releasing this vulnerability to the general public.

We cover the technical details of the attack in more detail in a separate blog post: HTTP/2 Rapid Reset: deconstructing the record-breaking attack.

How is Cloudflare and the industry thwarting this attack?

There is no such thing as a “perfect disclosure.” Thwarting attacks and responding to emerging incidents requires organizations and security teams to live by an assume-breach mindset