The Apache Software Foundation has issued crucial updates to address multiple vulnerabilities within the HTTP Server. Among these, a critical flaw could potentially lead to remote code execution (RCE), posing significant risks to server security. This vulnerability, identified as CVE-2026-23918 and carrying a CVSS score of 8.8, involves a double-free error in the HTTP/2 protocol handling. Affected systems include Apache HTTP Server version 2.4.66, with patches available in version 2.4.67.
Vulnerability Details and Discovery
Security experts Bartlomiej Dmitruk of Striga.ai and Stanislaw Strzalkowski from ISEC.pl discovered and reported the flaw. Dmitruk emphasized the critical nature of CVE-2026-23918, which can be exploited for denial-of-service (DoS) attacks and possible RCE. The vulnerability occurs within Apache httpd 2.4.66’s mod_http2, specifically in the stream cleanup process. It is triggered when a client sends an HTTP/2 HEADERS frame followed by a RST_STREAM with a non-zero error code, leading to memory issues.
Two callbacks, on_frame_recv_cb and on_stream_close_cb, are executed in sequence, both invoking the same cleanup path, resulting in a double-free scenario. This can cause system instability, making the DoS attack straightforward and effective on any default setup with mod_http2.
Exploitation and Impact
The denial-of-service aspect of this vulnerability is relatively easy to exploit, requiring only a single TCP connection and minimal setup. The server’s worker process crashes, dropping requests, and can be repeatedly targeted as long as the attacker sends data. For remote code execution, a more complex exploitation path is required, involving the Apache Portable Runtime with the mmap allocator, common in Debian-based systems and official Docker images for httpd.
A working proof of concept for RCE has been developed, utilizing a fake h2_stream structure placed at a freed memory address. This structure’s pool cleanup function redirects to system(), using the server’s scoreboard memory for stability. Although practical exploitation requires additional information leaks, such as system() addresses and scoreboard offsets, Dmitruk notes that successful execution is feasible under controlled conditions.
Mitigation and Recommendations
The MPM prefork configuration is not affected by this vulnerability, but given the widespread use of mod_http2 and HTTP/2 in production environments, the attack surface is considerable. Users are strongly advised to implement the latest patches to safeguard their systems against potential exploits. As HTTP/2 gains prevalence, ensuring server security through timely updates is crucial.
In conclusion, the CVE-2026-23918 vulnerability underscores the importance of maintaining up-to-date server software. The Apache Software Foundation’s prompt response with version 2.4.67 highlights the need for vigilance in server security management.
