CDN Tsunami Attack Abuses HTTP/3 Translation for Up to 350x DoS Amplification
Cybersecurity researchers have disclosed two denial-of-service (DoS) attacks that exploit how major content delivery networks (CDNs) convert client-facing HTTP/3 traffic into HTTP/1.1 requests, amplifying a low-bandwidth request stream by up to 350x against the origin server.
Cybersecurity researchers have disclosed two denial-of-service (DoS) attacks that exploit how major content delivery networks (CDNs) convert client-facing HTTP/3 traffic into HTTP/1.1 requests. The attacks, collectively named CDN Tsunami, were evaluated against six major CDNs: Alibaba, Baidu, Cloudflare, Amazon CloudFront, Fastly, and Tencent.
Attack Details
All six CDNs were found susceptible to the bandwidth variant and five to the connection variant, with Cloudflare unaffected by the latter because it buffers the complete request before opening a connection to the origin. The attack requires a website hosted on one of the six providers, with HTTP/3 serving at the edge, and no configuration changes on the website's part.
The 350x factor applies only to Alibaba, Baidu, and Tencent, which support the QPACK dynamic table. The maximum amplification factors measured using the QPACK static table are as follows:
| Provider | Amplification Factor | Dynamic Table Support |
|---|---|---|
| Baidu | 66.06x | supported |
| Alibaba | 65.8x | supported |
| Tencent | 54.08x | supported |
| Amazon CloudFront | 51.2x | no support |
| Cloudflare | 48.27x | no support |
| Fastly | 36.41x | no support |
HTTP/3 Bandwidth Amplification (HBA) and HTTP/3 Connection Amplification (HCA)
HBA leverages QPACK, the header compression format introduced with HTTP/3. Because HTTP/1.1 carries no equivalent mechanism, the CDN has to expand every small index value it receives back into a full raw header before forwarding the request. HCA targets connection capacity rather than bandwidth, and five of the six CDNs open an HTTP/1.1 connection to the origin as soon as they receive the HTTP/3 HEADERS frame.
Mitigations and Response
The mitigations put to the vendors are applied at the CDN, and include capping the size of any single header field entry inserted into the QPACK dynamic table, limiting how many times one dynamic table entry can be referenced within a single stream, and enforcing a maximum decompressed HTTP/1.1 request size. Tencent's deployed mitigations limit the number of CDN-to-origin connections and restrict the size of headers in the dynamic table.
The work is credited to researchers at the National University of Singapore, Fuzhou University, the University of Sheffield, and Johns Hopkins University. The paper does not report whether the attacks were retested after Baidu and Tencent deployed their mitigations, and does not say whether the attack code or the measurement framework will be published.
