Critical Apache HTTP2 Flaw CVE-2026-24663
The Apache Software Foundation has issued critical security updates for its HTTP Server to address multiple vulnerabilities, including a severe flaw that could allow attackers to execute arbitrary code remotely. The most serious vulnerability, tracked as CVE-2026-23918 with a CVSS score of 8.8, is a double free memory corruption bug in the HTTP/2 protocol handler. This flaw affects Apache HTTP Server version 2.4.66 and has been patched in the newly released version 2.4.67. The vulnerability was discovered and reported by security researchers Bartlomiej Dmitruk from Striga.ai and Stanislaw Strzalkowski from ISEC.pl.
The vulnerability exists in the mod_http2 module, specifically in the h2_mplx.c file, and occurs when a client sends certain HTTP/2 frames in rapid succession before the server has fully initialized its connection handling. This race condition causes the server to attempt freeing the same memory location twice, leading to memory corruption. According to Dmitruk, exploiting this flaw for denial of service is straightforward and requires minimal effort, just one TCP connection and two HTTP/2 frames with no authentication or special headers needed. A successful DoS attack causes the server to crash and restart, resulting in dropped requests during the recovery period.
While the denial of service attack is trivial to execute, achieving remote code execution is more complex but still feasible under certain conditions. The exploit requires a specific memory allocator configuration that is commonly found on Debian-based systems and the official Apache httpd Docker image. Dmitruk's team developed a proof-of-concept exploit for x86_64 systems that manipulates the freed memory by placing malicious structures and leveraging Apache's memory pools to inject shellcode. Though the exploit requires some knowledge of the target system's memory layout and success is not guaranteed on every attempt, testing showed it could work within minutes in vulnerable environments.
Dmitruk emphasized that while the MPM prefork configuration is not vulnerable to this particular issue, the widespread default inclusion of mod_http2 in Apache installations and the prevalence of HTTP/2 usage mean that a significant number of systems are potentially at risk. Organizations running affected versions should prioritize upgrading to Apache HTTP Server 2.4.67 immediately to mitigate the threat of both denial of service attacks and potential remote code execution.
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