Executive Summary

CVE-2026-27135 is a denial of service vulnerability in the nghttp2 library (versions up to 1.68.0) caused by missing internal state validation during session termination.

When the public APIs nghttp2_session_terminate_session or nghttp2_session_terminate_session2 are called—either by the application or internally upon detecting connection errors—the library incorrectly continues reading incoming data without proper state checks.

If a malformed frame causing a FRAME_SIZE_ERROR is received after this, it triggers an assertion failure that crashes the process.

This can happen in various scenarios involving specific HTTP/2 frames like ALTSVC, PRIORITY_UPDATE, or user-defined extension frames, especially when these frames are enabled explicitly.

The vulnerability was fixed in nghttp2 version 1.68.1 by adding the missing state validation to prevent these assertion failures.

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Impact Analysis

This vulnerability can cause a denial of service (DoS) condition by crashing the process using the nghttp2 library.

An attacker can exploit this by sending malformed HTTP/2 frames after session termination APIs are called, leading to an assertion failure and process crash.

The impact is availability-related, meaning the affected service or application may become unavailable or stop responding due to the crash.

No confidentiality or integrity impacts are associated with this vulnerability.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability is triggered by the nghttp2 library continuing to read incoming data after session termination APIs are called, leading to assertion failures when malformed frames causing FRAME_SIZE_ERROR are received.'}, {'type': 'paragraph', 'content': 'Detection involves monitoring for crashes or assertion failures in applications using nghttp2 versions prior to 1.68.1, especially when processing ALTSVC, PRIORITY_UPDATE, or user-defined extension frames.'}, {'type': 'paragraph', 'content': "Since the issue is internal to the nghttp2 library's frame processing, direct network detection commands are not provided in the resources."}, {'type': 'paragraph', 'content': 'However, you can check the version of nghttp2 installed on your system with commands like:'}, {'type': 'list_item', 'content': 'nghttp2 --version'}, {'type': 'list_item', 'content': 'dpkg -l | grep nghttp2 (on Debian/Ubuntu)'}, {'type': 'list_item', 'content': 'rpm -qa | grep nghttp2 (on RedHat/CentOS)'}, {'type': 'paragraph', 'content': 'Additionally, monitoring application logs for crashes or assertion failures related to nghttp2 when processing HTTP/2 frames may help detect exploitation attempts.'}] [1]

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Mitigation Strategies

The primary mitigation is to upgrade the nghttp2 library to version 1.68.1 or later, where the missing internal state validation has been added to prevent assertion failures.

No known workarounds exist for this vulnerability, so applying the patch or upgrading is strongly recommended.

If upgrading immediately is not possible, consider recompiling nghttp2 with assertions disabled (e.g., using the -DNDEBUG flag), which prevents assertion failures but does not fully mitigate the underlying issue.

Also, review and restrict the use of PRIORITY_UPDATE, ALTSVC, and user-defined extension frames if possible, as these frame types are involved in triggering the vulnerability.

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