CVE-2026-27135 in nghttp2
Summary
by MITRE • 03/18/2026
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 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.
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Analysis
by VulDB Data Team • 05/03/2026
The vulnerability described in CVE-2026-27135 affects the nghttp2 library, a widely-used implementation of HTTP/2 protocol in C programming language. This library serves as a critical component in modern web infrastructure, handling HTTP/2 connections between clients and servers. The flaw manifests in versions prior to 1.68.1 where the library exhibits problematic behavior during session termination scenarios, creating potential for system instability and denial of service conditions. The vulnerability specifically impacts the library's handling of connection termination through two primary application programming interfaces: `nghttp2_session_terminate_session` and `nghttp2_session_terminate_session2`. These APIs are designed to gracefully end HTTP/2 sessions, but their implementation contained a critical design flaw that allowed for improper state management during the termination process.
The technical root cause of this vulnerability stems from inadequate internal state validation within the nghttp2 library's session management system. When either of the termination APIs is invoked, the library should properly transition to a state where no further data processing occurs. However, the absence of proper validation allowed the system to continue reading incoming data streams even after session termination had been initiated. This behavior creates a dangerous scenario where the library remains in a partially initialized state, susceptible to processing malformed data frames that would normally be rejected. The vulnerability becomes particularly exploitable when the library encounters a FRAME_SIZE_ERROR condition, which triggers an assertion failure due to the inconsistent internal state. This assertion failure represents a critical point of system instability that can lead to application crashes or complete service outages.
The operational impact of CVE-2026-27135 extends beyond simple application crashes to encompass broader security and reliability concerns within HTTP/2 implementations. Attackers could potentially exploit this vulnerability by crafting malicious HTTP/2 frames that, when processed after session termination, trigger the assertion failure and cause denial of service conditions. The vulnerability affects systems that rely on nghttp2 for HTTP/2 protocol handling, including web servers, reverse proxies, and client applications that utilize HTTP/2 connections. Given that HTTP/2 is increasingly adopted across enterprise environments and web services, the potential for widespread impact is significant. The vulnerability's classification aligns with CWE-248, which addresses "Uncaught Exception" conditions, and represents a failure in proper error handling and state management within the library's session termination logic. This type of vulnerability can be particularly dangerous in high-availability systems where maintaining stable connection handling is critical for service continuity.
The remediation for this vulnerability requires immediate upgrade to nghttp2 version 1.68.1 or later, which implements the necessary state validation to prevent the assertion failure condition. The fix addresses the underlying issue by ensuring proper internal state management during session termination, preventing the library from continuing to process data after termination has been initiated. Security teams should prioritize patching affected systems, particularly those handling high volumes of HTTP/2 traffic or serving critical business functions. Organizations using custom implementations or modified versions of nghttp2 should verify that their specific configurations are not susceptible to similar issues. The vulnerability demonstrates the importance of proper state validation in network protocol libraries and highlights the need for comprehensive testing of edge cases during session management operations. While no known workarounds exist for this specific vulnerability, system administrators should monitor for potential exploitation attempts and maintain robust monitoring for application stability indicators that might signal the occurrence of such issues.