Critical Apache HTTP/2 Flaw (CVE-2026-23918) Enables DoS and Potential RCE

The Apache Software Foundation has released security updates to address a critical vulnerability in its HTTP Server that could lead to both denial-of-service (DoS) attacks and potential remote code execution (RCE). The vulnerability, tracked as CVE-2026-23918, carries a CVSS score of 8.8 and affects Apache HTTP Server version 2.4.66, with fixes available in version 2.4.67.

Understanding the Technical Vulnerability

CVE-2026-23918 is a double-free vulnerability in the mod_http2 module, specifically in the stream cleanup path of h2_mplx.c. The issue occurs when a client sends an HTTP/2 HEADERS frame immediately followed by a RST_STREAM frame with a non-zero error code on the same stream, before the multiplexer has registered the stream.

"Two nghttp2 callbacks then fire in sequence, on_frame_recv_cb for the RST and on_stream_close_cb for the close, and both end up calling h2_mplx_c1_client_rst -> m_stream_cleanup, which pushes the same h2_stream pointer onto the spurge cleanup array twice," explained Bartlomiej Dmitruk, co-founder of Striga.ai, who discovered the vulnerability along with Stanislaw Strzalkowski from ISEC.pl.

When c1_purge_streams later iterates spurge and calls h2_stream_destroy -> apr_pool_destroy on each entry, the second call attempts to free memory that has already been freed, leading to the vulnerability.

Exploitation Scenarios

The vulnerability presents two distinct exploitation paths:

1. Denial-of-Service: This path is straightforward and affects any default deployment with mod_http2 and a multi-threaded MPM. "The DoS is trivial and works on any default deployment with mod_http2 and a multi-threaded MPM," Dmitruk noted. "One TCP connection, two frames, no authentication, no special headers, no specific URL, and the worker crashes. Apache respawns it, but every request on the crashed worker is dropped, and the pattern can be sustained as long as the attacker keeps sending."

2. Remote Code Execution: The RCE path is more complex and requires specific conditions. "The RCE path requires an Apache Portable Runtime (APR) with the mmap allocator, which is the default on Debian-derived systems and on the official httpd Docker image," Dmitruk explained. "The chain places a fake h2_stream struct at the freed virtual address via mmap reuse, points its pool cleanup function to system(), and uses Apache's scoreboard memory as a stable container for the fake structures and the command string."

The researcher noted that while the RCE chain requires certain conditions, it's practical in real-world scenarios. "The usual caveats apply: practical exploitation requires an info leak for system() and the scoreboard offsets, and the heap spray is probabilistic, but in lab conditions execution lands in minutes."

Affected Systems and Mitigation

The vulnerability affects Apache HTTP Server 2.4.66 and has been fixed in version 2.4.67. Dmitruk pointed out that the MPM prefork is not affected by this flaw, but the attack surface is significant because mod_http2 ships in default builds and HTTP/2 is widely enabled in production deployments.

For organizations running Apache HTTP Server, the recommended mitigation is straightforward: upgrade to version 2.4.67 or later. The Apache Software Foundation has made the security announcement available with detailed information about the vulnerability and the fixes.

Broader Implications

This vulnerability underscores the ongoing challenges in implementing complex protocols like HTTP/2 securely. As web applications increasingly rely on HTTP/2 for performance benefits, the attack surface for protocol-level vulnerabilities expands.

"The discovery of this vulnerability highlights the importance of thorough testing of protocol implementations," Dmitruk added. "HTTP/2 introduces significant complexity compared to HTTP/1.1, and these subtle timing and state management issues can have severe consequences."

For organizations that cannot immediately upgrade, mitigating the vulnerability would involve disabling HTTP/2 support by removing or commenting out the mod_http2 module configuration. However, this approach comes with performance implications and may not be feasible for all production environments.

Industry Response

The security community has responded swiftly to this vulnerability, with researchers and security teams analyzing the proof-of-concept code and developing detection mechanisms. Organizations are advised to monitor their systems for any signs of exploitation, particularly unusual worker crashes or suspicious network traffic patterns that might indicate exploitation attempts.

The vulnerability serves as a reminder of the critical importance of prompt patching for web server software, especially when the vulnerabilities can lead to remote code execution. In an era where web applications are primary targets for attackers, maintaining up-to-date server software is a fundamental security practice.

For administrators running Apache HTTP Server, the priority should be applying the security updates as soon as possible, as the combination of DoS and RCE capabilities makes this a particularly dangerous vulnerability in environments where availability and integrity of web services are critical.