CDN Tsunami attack amplifies DoS 350x via HTTP/3
CDN Tsunami attack abuses HTTP/3 translation for massive DoS amplification
Researchers from the National University of Singapore, Fuzhou University, the University of Sheffield, and Johns Hopkins University have disclosed two denial-of-service attack techniques -- collectively named "CDN Tsunami" -- that exploit a fundamental protocol mismatch in how major content delivery networks handle HTTP/3 traffic. The attacks amplify a low-bandwidth request stream by up to 350x against the origin server.
The techniques, evaluated against Alibaba, Baidu, Cloudflare, Amazon CloudFront, Fastly, and Tencent, exploit the fact that CDNs speak HTTP/3 to the browser but only HTTP/1.1 to the origin website. The first method, HTTP/3 Bandwidth Amplification, leverages QPACK header compression: a request costing the attacker a few bytes on the wire forces the CDN to expand it into a full raw header before forwarding, consuming up to 100 Mbps at the origin while the attacker stays below 500 Kbps. The second method, HTTP/3 Connection Amplification, opens multiple backend TCP connections per HTTP/3 stream, exhausting origin server connection pools.
The 350x amplification factor applies to Alibaba, Baidu, and Tencent, which support the QPACK dynamic table. Maximum factors for CloudFront, Cloudflare, and Fastly ranged from 36x to 51x using the static table alone. Researchers identified over 42,000 potentially vulnerable subdomains across the six providers. No CVE identifiers have been assigned and no in-the-wild exploitation has been reported. Baidu and Tencent confirmed the findings and deployed mitigations; the other four vendors are still evaluating. The research is scheduled for presentation at the Symposium on Reliable Distributed Systems in Rome in September 2026.