CDN Tsunami Attack Exploits HTTP/3 Flaw for 350x DoS Amplification

Cybersecurity researchers have disclosed two denial-of-service (DoS) techniques collectively dubbed “CDN Tsunami” that weaponize the protocol translation layer inside major content delivery networks. The attacks target how CDNs terminate client-facing HTTP/3 traffic at the edge but forward HTTP/1.1 requests to the origin server, creating an asymmetry that lets a single low-bandwidth attacker stream amplify into a 350x bandwidth storm against the backend. All six providers evaluated—Alibaba, Baidu, Cloudflare, Amazon CloudFront, Fastly, and Tencent—were found vulnerable to the bandwidth variant, while five of the six (Cloudflare excepted) were also susceptible to the connection variant due to its practice of buffering the complete request before opening an origin connection. Notably, the attack requires zero configuration changes from website owners; any site served with HTTP/3 at the edge is a potential target. Security teams can audit their own origin exposure using a port scanner to confirm which ports and protocols are publicly reachable, since the attacker’s payload is directed entirely at the CDN-to-origin hop.

The two techniques, named HTTP/3 Bandwidth Amplification (HBA) and HTTP/3 Connection Amplification (HCA), both exploit the same root cause: a deployment gap where CDNs speak modern QUIC-based HTTP/3 to browsers but fall back to legacy HTTP/1.1 when talking to origin servers. HBA specifically leverages QPACK, the header compression scheme bundled with HTTP/3. Because HTTP/1.1 has no equivalent header compression, the CDN must decompress and expand every indexed reference back into a full raw header before forwarding it. An attacker sending a few bytes over the wire can therefore force the origin to absorb a dramatically larger payload after expansion. In practice, attacker-side bandwidth stayed below 500 Kbps against Alibaba, Baidu, and Tencent, while origin-side consumption exceeded 100 Mbps, a ratio that holds even when CDN-to-origin sessions are encrypted, which is why verifying strong SSL/TLS configurations remains a baseline defense.

The 350x amplification ceiling applies only to Alibaba, Baidu, and Tencent, the three providers that support QPACK’s dynamic table with roughly 64 concurrent streams and 4KB tables allowing 3,072-byte entries. Cloudflare, CloudFront, and Fastly registered more modest ratios ranging from 36.41x to 51.2x. To trigger the dynamic table variant, the attacker first sends a single HTTP/3 request carrying an oversized header—which the CDN caches in its table—and then references that entry repeatedly using tiny index values, multiplying each small inbound packet into a much larger outbound header set against the origin. No CVEs have been assigned, and no in-the-wild exploitation has been observed, but the research paper was disclosed responsibly and Baidu and Tencent have already deployed proposed fixes. Every mitigation sits at the CDN layer rather than at the origin website, meaning operators of HTTP/3-enabled properties should confirm their provider’s remediation status and run a broader privacy and security checkup to map every fronted asset before assuming protection.