CVE-2026-27135
Nghttp2 ≤ 1.68.1
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-27135 is a high-severity Reachable Assertion (CWE-617) vulnerability in Nghttp2 Nghttp2. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked in the top 47% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-8 (Security and Privacy Engineering Principles) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-27135 is a vulnerability in nghttp2, an open-source C implementation of the Hypertext Transfer Protocol version 2 (HTTP/2). In versions prior to 1.68.1, the library fails to properly validate internal state after the user-facing APIs `nghttp2_session_terminate_session` or `nghttp2_session_terminate_session2` are invoked, either directly by the application or internally due to a connection error. This allows the library to continue reading incoming data, and a subsequent malformed frame triggering a FRAME_SIZE_ERROR results in an assertion failure, linked to CWE-617 (Reachable Assertion).
A remote, unauthenticated attacker can exploit this vulnerability over the network with low complexity and no user interaction required, as indicated by its CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H). By sending crafted HTTP/2 traffic to a vulnerable nghttp2-based service, the attacker can induce the termination session state and follow with a malformed frame, causing the library to crash via assertion failure and leading to denial of service through high availability impact.
The nghttp2 project addressed this in version 1.68.1 by adding the missing internal state validation to prevent the assertion failure after session termination. Relevant advisories include the GitHub Security Advisory GHSA-6933-cjhr-5qg6 and a commit at https://github.com/nghttp2/nghttp2/commit/5c7df8fa815ac1004d9ecb9d1f7595c4d37f46e1, with discussion on the oss-security mailing list at http://www.openwall.com/lists/oss-security/2026/03/20/3. No known workarounds exist.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-12919
Vulnerability Data
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 `nghttp2_session_terminate_session` or `nghttp2_session_terminate_session2` is called by the application. They might…
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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 FRAME_SIZE_ERROR causes assertion failure. nghttp2 v1.68.1 adds missing state validation to avoid assertion failure. No known workarounds are available.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation finds reachable assertions during development.
Security engineering principles discourage use of assertions for handling untrusted input.
Validating untrusted inputs structurally prevents attacker data from reaching and triggering assertions.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly prevent unsafe assertions from being coded in reachable paths.
Runtime monitoring of software can detect assertion-triggered crashes as adverse events.
Vulnerability identification processes can discover and record reachable-assertion flaws before deployment.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing in development can detect reachable assertions before release, reducing the likelihood of exploitation.
Secure development lifecycle mandates defensive coding and input validation that prevent reachable assertions from being triggered by untrusted data.
Application security requirements can specify that assertions must not be reachable from attacker-controlled inputs.
Secure architecture principles discourage the use of assertions for runtime error handling that an attacker could exploit.
Secure coding standards explicitly ban the use of assert() or equivalent statements that can be triggered by external input.