Two denial-of-service techniques that exploit the protocol conversion inside major content delivery networks can turn a trickle of attack traffic into a flood at the origin. Dubbed “CDN Tsunami,” the pair amplify a low-bandwidth request stream by as much as 350x, the researchers found.
The team tested the attacks against Alibaba, Baidu, Cloudflare, Amazon CloudFront, Fastly, and Tencent. All six were susceptible to the bandwidth variant, and five to the connection variant; Cloudflare resists the second because it buffers the full request before opening a backend connection.
The bandwidth technique exploits QPACK, HTTP/3’s header compression format. HTTP/1.1 has nothing comparable, so the edge must blow small index values back up into full raw headers. A request that costs the attacker a few bytes on the wire ends up costing the origin the fully decompressed size. The connection variant opens a backend TCP connection per multiplexed stream before the request body arrives, then keeps those connections hanging with slow data.
Measured at the origin, bandwidth use passed 100 Mbps throughout the tests. On the attacker side, less than 500 Kbps sufficed against the three CDNs with dynamic-table support, and under 5 Mbps against the rest. The strongest amplification, up to 350x, was measured on Alibaba, Baidu, and Tencent. Probing 151,685 subdomains across the six providers surfaced 42,330 that answered an HTTP/3 request, a group the researchers marked as potentially vulnerable.
No CVE numbers accompany the disclosure, and the authors report no attacks in the wild. Baidu and Tencent acknowledged the work and shipped fixes. The remaining providers were still weighing mitigations, all of which land at the CDN rather than the origin. The work is due to be presented at a reliability symposium in Rome in late September.
