Last updated 2 July 2026

https://github.com/nghttp2/nghttp2/security/advisories/GHSA-6933-cjhr-5qg6 advises: Impact ------ nghttp2 library stops reading the incoming data when user facing public API nghttp2sessionterminatesession or nghttp2sessionterminatesession2 is called by the application. They might be called internally by the library when it detects the situation that is subject to connection error.

Due to the missing internal state validation, the library keeps reading the rest of the data after one of those APIs is called. Then receiving a malformed frame that causes FRAMESIZEERROR causes assertion failure.

nghttp2sessionterminatesession and nghttp2sessionterminatesession2 are used interchangeably in the rest of this document.

The detailed execution paths that lead to assertion failure are:

- nghttp2sessionterminatesession is called from nghttp2oninvalidframerecvcallback when processing incoming ALTSVC (RFC 7838) frame, and then a malformed frame that causes FRAMESIZEERROR is received.

- nghttp2sessionterminatesession is called from nghttp2onframerecvcallback when processing PRIORITYUPDATE, ALTSVC, or user-defined extension frame, and then a malformed frame that causes FRAMESIZEERROR is received.

- nghttp2sessionterminatesession is called from nghttp2onextensionchunkrecvcallback when processing user-defined extension frame, and then a malformed frame that causes FRAMESIZEERROR is received.

- nghttp2sessionterminatesession is called from nghttp2unpackextensioncallback when processing user-defined extension frame, and then a malformed frame that causes FRAMESIZEERROR is received.

- The malformed PRIORITYUPDATE frame (e.g., streamid == 0) is received, and then a malformed frame that causes FRAMESIZEERROR is received.

For PRIORITYUPDATE and ALTSVC frames, they must be explicitly enabled via nghttp2optionsetbuiltinrecvextensiontype to be affected.

For user-defined extension frames, they must be explicitly enabled via nghttp2optionsetuserrecvextensiontype to be affected.

For the builds that disable assert macro (e.g, -DNDEBUG), it does not hit assertion failure, but it might continue reading incoming data, which itself does not cause any vulnerability as far as we know. We still strongly encourage to apply the patch for those builds as well.

The reporter found the cases where ALTSVC is involved. The other cases have been investigated and discovered by the nghttp2 development team.

Patches ------- nghttp2 v1.68.1 adds missing state validation to avoid assertion failure.

Workarounds ----------- There is no workaround.

References ---------- https://github.com/nghttp2/nghttp2/commit/5c7df8fa815ac1004d9ecb9d1f7595c4d37f46e1 fixes this vulnerability.

Severity -------- High 7.5 / 10 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVE ID ------ CVE-2026-27135

Credits ------- @AndrewMohawk

nghttp2 is an implementation of the Hypertext Transfer Protocol version 2 in C. Prior to version 1.68.1, the nghttp2 library stops reading the incoming data when user facing public API nghttp2sessionterminatesession or nghttp2sessionterminatesession2 is called by the application. They might be called internally by the library when it detects the situation that is subject to connection error. Due to the missing internal state validation, the library keeps reading the rest of the data after one of those APIs is called. Then receiving a malformed frame that causes FRAMESIZEERROR causes assertion failure. nghttp2 v1.68.1 adds missing state validation to avoid assertion failure. No known workarounds are available.

nghttp2 Denial of service: Assertion failure due to the missing state validation

Last updated 24 July 2024

HTTP/2 Rapid reset attack The HTTP/2 protocol allows clients to indicate to the server that a previous stream should be canceled by sending a RSTSTREAM frame. The protocol does not require the client and server to coordinate the cancellation in any way, the client may do it unilaterally. The client may also assume that the cancellation will take effect immediately when the server receives the RSTSTREAM frame, before any other data from that TCP connection is processed.

Abuse of this feature is called a Rapid Reset attack because it relies on the ability for an endpoint to send a RSTSTREAM frame immediately after sending a request frame, which makes the other endpoint start working and then rapidly resets the request. The request is canceled, but leaves the HTTP/2 connection open.

The HTTP/2 Rapid Reset attack built on this capability is simple: The client opens a large number of streams at once as in the standard HTTP/2 attack, but rather than waiting for a response to each request stream from the server or proxy, the client cancels each request immediately.

The ability to reset streams immediately allows each connection to have an indefinite number of requests in flight. By explicitly canceling the requests, the attacker never exceeds the limit on the number of concurrent open streams. The number of in-flight requests is no longer dependent on the round-trip time (RTT), but only on the available network bandwidth.

In a typical HTTP/2 server implementation, the server will still have to do significant amounts of work for canceled requests, such as allocating new stream data structures, parsing the query and doing header decompression, and mapping the URL to a resource. For reverse proxy implementations, the request may be proxied to the backend server before the RSTSTREAM frame is processed. The client on the other hand paid almost no costs for sending the requests. This creates an exploitable cost asymmetry between the server and the client.

Multiple software artifacts implementing HTTP/2 are affected. This advisory was originally ingested from the swift-nio-http2 repo advisory and their original conent follows.

swift-nio-http2 specific advisory swift-nio-http2 is vulnerable to a denial-of-service vulnerability in which a malicious client can create and then reset a large number of HTTP/2 streams in a short period of time. This causes swift-nio-http2 to commit to a large amount of expensive work which it then throws away, including creating entirely new Channels to serve the traffic. This can easily overwhelm an EventLoop and prevent it from making forward progress.

swift-nio-http2 1.28 contains a remediation for this issue that applies reset counter using a sliding window. This constrains the number of stream resets that may occur in a given window of time. Clients violating this limit will have their connections torn down. This allows clients to continue to cancel streams for legitimate reasons, while constraining malicious actors.

Envoy is a cloud-native high-performance edge/middle/service proxy. Envoy’s HTTP/2 codec may leak a header map and bookkeeping structures upon receiving RSTSTREAM immediately followed by the GOAWAY frames from an upstream server. In nghttp2, cleanup of pending requests due to receipt of the GOAWAY frame skips de-allocation of the bookkeeping structure and pending compressed header. The error return [code path] is taken if connection is already marked for not sending more requests due to GOAWAY frame. The clean-up code is right after the return statement, causing memory leak. Denial of service through memory exhaustion. This vulnerability was patched in versions(s) 1.26.3, 1.25.8, 1.24.9, 1.23.11.