The HTTP/2 CONTINUATION Flood is a denial-of-service attack that exploits how some implementations handle unfinished header blocks. It can consume CPU or memory and, in some cases, crash a server. Security researcher Bartek Nowotarski described it as potentially more dangerous than Rapid Reset in some situations, but no comparable attack-volume measurement proves it is universally more severe.
What is an HTTP/2 CONTINUATION Flood?
HTTP/2 sends request headers in header blocks. A block can span a HEADERS or PUSH_PROMISE frame and one or more CONTINUATION frames; the receiver knows the block is complete when it receives the END_HEADERS flag. CERT/CC’s VU#421644 says some implementations do not adequately limit CONTINUATION frames within a stream.
An attacker can start a header block and keep sending CONTINUATION frames without finishing it. A vulnerable implementation may continue processing or storing the data, consuming resources until service degrades or, depending on its behavior, runs out of memory and crashes. Because the header block remains incomplete, the malicious traffic may never become a valid, completed HTTP request.
Why is it being compared with Rapid Reset?
Both attacks exploit the cost of HTTP/2 processing, but they use different frame patterns:
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| Attack | Frame behavior | Targeted processing cost |
|---|---|---|
| CONTINUATION Flood | Keeps a header block open by sending CONTINUATION frames without END_HEADERS. | Header-block processing or storage in implementations that do not adequately constrain the frames. |
| Rapid Reset (CVE-2023-44487) | Opens many streams and immediately cancels them with reset behavior. | Work performed by the server for requests that are then canceled. |
Google Cloud reported that a Rapid Reset campaign peaked above 398 million requests per second in 2023. That figure describes Rapid Reset; it is not a CONTINUATION Flood measurement. The available reporting does not establish a head-to-head attack-volume comparison. SecurityWeek’s April 2024 coverage attributed the view that CONTINUATION Flood could be more dangerous in some cases to Nowotarski, including the possibility that a single machine could disrupt sites and APIs. That is a qualified risk assessment, not proof of a universal severity ranking.
Which HTTP/2 implementations may be affected?
This is an implementation vulnerability, not a flaw that automatically affects every HTTP/2 server. CERT/CC’s VU#421644 identifies these implementation-specific CVEs:
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- Apache HTTP Server: CVE-2024-27316
- Apache Traffic Server: CVE-2024-31309
- Envoy: CVE-2024-30255
- nghttp2: CVE-2024-28182
- Go net/http and golang.org/x/net/http2: CVE-2023-45288
The same CERT/CC vendor table records products, including Jetty and Vert.x, whose vendors said they were not affected. Check the advisory for the specific product and verify the server, proxy, and HTTP/2 library versions actually deployed; the presence of HTTP/2 alone does not establish exposure.
CERT/CC released VU#421644 on April 3, 2024, and last revised it on July 19, 2024. It is a useful starting point for affected-product information, but it is not a complete current patch matrix. Use the relevant project’s or vendor’s current advisory to identify the fixed release for your deployment; the CVE list alone does not tell you which version to install.
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How should operators respond?
- Inventory HTTP/2 endpoints. Identify internet-facing services that negotiate HTTP/2, including the web server, reverse proxy, and any HTTP/2 library they use.
- Check vendor guidance and patch. Match each deployed implementation and version to its current project or vendor advisory, then apply the specified fix. Do not assume one product’s status applies to another component in the request path.
- Monitor connections and frames. Look for abnormal connection and frame behavior. Since malicious traffic may not complete a valid HTTP request, ordinary request logs may not show the full pattern; raw HTTP traffic analysis may be needed.
- Use DDoS controls as another layer. CERT-EU recommends DDoS protection mechanisms as a longer-term measure in its Rapid Reset advisory. Such protection can complement, but should not replace, patching vulnerable software.
If considering a temporary restriction on HTTP/2, assess where it would be enforced and what it would mean for clients and service performance; the cited material does not establish a universal vendor-specific workaround. Confirm any emergency mitigation with the relevant vendor or project.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the standards do—and do not—say
The IETF HTTP Working Group said this issue is not a vulnerability in the HTTP/2 specification. RFC 9113 already warns about denial of service from large numbers of small or empty frames. The practical risk described by CERT/CC is that particular implementations may fail to constrain CONTINUATION frames adequately.
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