The Apache Software Foundation (ASF) has issued critical security updates for its ubiquitous HTTP Server, patching a severe vulnerability that could allow attackers to execute arbitrary code on vulnerable systems. This update addresses CVE-2026-23918, a high-severity flaw discovered by security researchers that impacts how the Apache HTTP Server handles the HTTP/2 protocol.
The vulnerability, tracked as CVE-2026-23918 and assigned a CVSS score of 8.8, represents a “double free” condition within the HTTP/2 protocol handling. This could potentially be leveraged for remote code execution (RCE). The issue was identified in Apache HTTP Server version 2.4.66 and has been resolved in the newly released version 2.4.67. Security researchers Bartlomiej Dmitruk of Striga.ai and Stanislaw Strzalkowski of ISEC.pl are credited with discovering and reporting this significant security flaw.
Apache HTTP Server Faces Critical Remote Code Execution Vulnerability
The severity of CVE-2026-23918 cannot be overstated, as confirmed by Bartlomiej Dmitruk. He explained that the vulnerability can be exploited to achieve both denial-of-service (DoS) conditions and, more critically, remote code execution (RCE). This means attackers could potentially disrupt server operations or gain unauthorized control over affected systems.
According to Dmitruk, the vulnerability lies within the stream cleanup path of the `h2_mplx.c` file in Apache’s `mod_http2` module. It is triggered when a client sends an HTTP/2 HEADERS frame immediately followed by an RST_STREAM frame with a non-zero error code on the same stream, prior to the multiplexer registering the stream. This sequence causes two callbacks within the `nghttp2` library to fire consecutively. Both callbacks invoke a cleanup function that inadvertently pushes the same stream pointer onto a cleanup array twice. When the system later attempts to clean up these entries, it encounters a double free condition, leading to memory corruption.
Dmitruk further elaborated on the exploitability of the vulnerability. A denial-of-service attack is described as “trivial,” requiring only one TCP connection and two frames, with no need for authentication or specific headers. This can crash the server worker, causing requests to be dropped, and the pattern can be sustained by an attacker. The remote code execution path, while more complex, is also feasible. It requires the Apache Portable Runtime (APR) to be configured with the mmap allocator, which is the default setting on many popular Linux distributions, including Debian-derived systems and the official httpd Docker image.
The proof-of-concept developed by Dmitruk demonstrates that RCE is achievable on x86_64 architectures. The exploit involves placing a fake `h2_stream` structure at the freed virtual address through memory mapping and redirecting its pool cleanup function to execute the `system()` command. The server’s scoreboard memory, which remains at a fixed address throughout the server’s lifecycle even with Address Space Layout Randomization (ASLR) enabled, is utilized as a stable location for these fake structures and the command string. While practical exploitation necessitates an information leak to ascertain memory offsets for `system()` and the scoreboard, and heap spraying can be probabilistic, proof-of-concept demonstrations have shown successful execution within minutes under laboratory conditions.
It is important to note that the `mpm_prefork` module is not affected by this specific flaw. However, the attack surface for CVE-2026-23918 is substantial. The `mod_http2` module is included in default Apache HTTP Server builds, and HTTP/2 is widely enabled in production environments. The widespread use of Apache HTTP Server in serving web content makes this vulnerability a significant concern for web server security.
Mitigation and Future Outlook
Given the critical nature of CVE-2026-23918, users of Apache HTTP Server are strongly advised to update to version 2.4.67 or later as a matter of urgency. Applying the latest security patches is the most effective measure to protect against potential exploitation of this remote code execution vulnerability. Organizations should prioritize this update to ensure the ongoing security and integrity of their web infrastructure.