Critical Apache HTTP/2 Race Condition Enables Remote Code Execution
A severe synchronization flaw in mod_http2 allows unauthenticated attackers to corrupt heap memory.
1Critical Synchronization Flaw Identified
A critical vulnerability, tracked as CVE-2026-119618, affects the Apache HTTP Server's mod_http2 module. The flaw originates from a race condition within the assign_headers() function during concurrent thread execution. This synchronization error allows remote, unauthenticated attackers to trigger heap memory corruption, potentially leading to arbitrary code execution on the server.
2Discovery and Reporting Timeline
The vulnerability was independently identified by Anthropic's OSS Scanner and Google Cloud’s Vulnerability Research team. Anthropic reported the issue on October 26, while Google researchers followed last week. Although the flaw is rated 9.8 on the CVSS scale, no public exploit currently exists, and no active exploitation has been reported.
3Affected Versions and Software
The flaw impacts Apache HTTP Server versions 2.4.55 through 2.4.70. Additionally, standalone mod_h2 builds from GitHub are affected from version 2.0.8 through 2.0.42. Servers are only vulnerable if the HTTP/2 protocol is explicitly enabled via the Protocols directive, using either h2 for TLS or h2c for plain TCP connections.
4Remediation and Patching Guidance
Administrators should upgrade to Apache HTTP Server version 2.4.71, which addresses the synchronization issue. For those using standalone mod_h2 builds, no fixed release is currently available. If immediate patching is not feasible, administrators must disable HTTP/2 support by restricting the Protocols directive to http/1.1 in the server configuration files.
5Platform-Specific Security Assessments
Red Hat reports that its Enterprise Linux 9 and 10 distributions include specific memory allocator configurations that make exploitation significantly harder, leading them to rate the flaw as Important rather than Critical. Red Hat Enterprise Linux 8 and earlier versions do not contain the vulnerable code and remain unaffected by this flaw.
6Cloud Environment Risk Factors
Wiz data indicates that 64% of cloud environments contain resources vulnerable to this flaw, with 51% having publicly exposed instances. In containerized or virtualized setups, a compromised Apache instance could serve as an entry point for lateral movement, potentially exposing adjacent microservices or sensitive cloud metadata endpoints to unauthorized threat actors.
7Configuration and Scope Verification
The vulnerability is not limited to server-wide settings; it can be enabled for individual virtual hosts. Administrators must audit both global server configurations and every virtual host configuration to ensure the h2 and h2c protocols are removed. Relying solely on global settings may leave specific virtual hosts exposed to potential attacks.
8Broader Security Context
This incident highlights the persistent risks associated with complex module synchronization in web servers. While previous HTTP/2 flaws like CVE-2026-23918 were addressed in May, the current issue demonstrates that even updated systems remain susceptible to new memory corruption vectors. Continuous monitoring of protocol-specific modules is essential for maintaining robust server security.