In late August 2023, a significant cybersecurity threat emerged as Amazon Web Services (AWS), Cloudflare, and Google jointly reported the detection of a series of highly disruptive distributed denial-of-service (DDoS) attacks. These attacks were unique in their exploitation of a newly discovered technique known as the HTTP/2 Rapid Reset Zero-Day Vulnerability.
Furthermore, the severity of these attacks prompted a coordinated response from these tech giants, as they took immediate steps to mitigate the record-breaking DDoS onslaught. The vulnerability responsible for this wave of attacks has been identified as CVE-2023-44487, carrying a CVSS score of 7.5 out of 10, underscoring its significant threat level.
These DDoS attacks, classified as layer 7 attacks, unleashed a staggering volume of requests per second (RPS) on their targets. While Google’s cloud infrastructure experienced peak attack rates of 398 million RPS, AWS and Cloudflare were not spared either, with attack volumes exceeding 155 million and 201 million RPS, respectively. The root of these attacks lay in a zero-day flaw within the HTTP/2 protocol, which is designed to multiplex requests over a single TCP connection using concurrent streams.
The crux of the HTTP/2 Rapid Reset attack lies in its ability to send and cancel requests in rapid succession. This technique circumvents the server’s concurrent stream maximum, effectively overloading the server without reaching its configured threshold.
By initiating a multitude of HTTP/2 streams and rapidly canceling them at scale, threat actors can overwhelm websites, potentially taking them offline. What’s particularly concerning is that these attacks can be executed using a relatively modest-sized botnet, as observed by Cloudflare, with as few as 20,000 machines.
As HTTP/2 is a prevalent protocol, used by 35.6% of all websites, the vulnerability poses a widespread threat. Google Cloud reported the emergence of multiple variants of the Rapid Reset attacks, which, while not as effective as the initial version, were still more efficient than standard HTTP/2 DDoS attacks. This evolving threat landscape has prompted independent advisories, such as F5, to urge customers to update their NGINX configuration to limit concurrent streams and persist HTTP connections, emphasizing the urgency of proactive measures to defend against these threats.
Threat actors now possess a powerful tool that has the potential to exploit and disrupt systems at an unprecedented scale, signaling a race between defenders and attackers to address this critical vulnerability. Organizations are advised to assume that their systems will be tested and take immediate action to enhance their protection.