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

The Apache Software Foundation (ASF) has released security updates to address several security vulnerabilities in the HTTP Server, including a severe vulnerability that could potentially lead to remote code execution (RCE).

The vulnerability, tracked as CVE-2026-23918 (CVSS score: 8.8), has been described as a case of "double free and possible RCE" in the HTTP/2 protocol handling. This issue affects Apache HTTP Server 2.4.66 and has been addressed in version 2.4.67.

Striga.ai co-founder Bartlomiej Dmitruk and ISEC.pl researcher Stanislaw Strzalkowski have been credited with discovering and reporting the vulnerability.

When reached for comment, Dmitruk told The Hacker News via email that the severity of CVE-2026-23918 is critical, as it can be exploited to achieve denial-of-service (DoS) and RCE. Additional details of the vulnerability are below -

CVE-2026-23918 is a double-free in Apache httpd 2.4.66 mod_http2, specifically in the stream cleanup path of h2_mplx.c. The bug triggers when a client sends an HTTP/2 HEADERS frame immediately followed by RST_STREAM 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. When c1_purge_streams later iterates spurge and calls h2_stream_destroy -> apr_pool_destroy on each entry, the second call hits memory that has already been freed.

The DoS, Dmitruk added, is trivial and works on any default deployment with mod_http2 and a multi-threaded MPM, whereas 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 further explained -

The first is denial-of-service, which is trivial: 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.

The second outcome is remote code execution, and we built a working proof of concept on x86_64. 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 scoreboard sits at a fixed address for the lifetime of the server, even with ASLR, which is what makes the RCE path practical. 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.

Dmitruk also pointed out that the MPM prefork is not affected by the flaw. However, the researcher cautioned that the attack surface is large as mod_http2 ships in default builds and HTTP/2 is widely enabled in production deployments. In light of the severity of the flaw, users are advised to apply the latest fixes for optimal protection.