CDN Tsunami Attacks Exploit HTTP/3 Translation for Up to 350x DoS Amplification

Cybersecurity researchers have disclosed two denial-of-service (DoS) attack techniques, collectively named “CDN Tsunami,” that exploit how major content delivery networks (CDNs) convert client-facing HTTP/3 traffic into HTTP/1.1 requests to origin servers. The attacks can amplify a low-bandwidth request stream by up to 350x against the origin, potentially overwhelming it.

The attacks were evaluated against Alibaba, Baidu, Cloudflare, Amazon CloudFront, Fastly, and Tencent. All six were found susceptible to the bandwidth variant, and five to the connection variant, with Cloudflare unaffected by the latter due to its buffering of complete requests before opening origin connections. The 350x amplification factor applies only to Alibaba, Baidu, and Tencent, which support the QPACK dynamic table. No CVE identifiers have been assigned, and no exploitation in the wild has been reported.

The two techniques are HTTP/3 Bandwidth Amplification (HBA) and HTTP/3 Connection Amplification (HCA). HBA leverages QPACK, the header compression format in HTTP/3, where the CDN expands small index values into full raw headers when forwarding to HTTP/1.1 origins. HCA targets connection capacity by opening multiple backend connections per HTTP/3 stream and keeping them open with slow data transmission, exhausting origin connection limits.

Baidu and Tencent confirmed the reports and deployed proposed fixes, while other vendors acknowledged the disclosure. The researchers suggest mitigations such as capping QPACK dynamic table entry sizes, limiting references, buffering complete requests, and enforcing connection limits. The work is credited to researchers from the National University of Singapore, Fuzhou University, the University of Sheffield, and Johns Hopkins University, and is due to be presented at the Symposium on Reliable Distributed Systems in Rome, September 22-24, 2026.