CVE-2026-23918: 
Apache HTTP Server vulnerability analysis and mitigation

CVE-2026-23918 is a Double Free vulnerability with possible Remote Code Execution (RCE) in Apache HTTP Server's HTTP/2 protocol implementation (mod_http2), specifically triggered on early connection reset. It affects exclusively Apache HTTP Server version 2.4.66, and was fixed in version 2.4.67. The vulnerability was reported on December 10, 2025, fixed in the Apache codebase on December 11, 2025, and publicly disclosed on May 4, 2026. It carries a CVSS v3.1 base score of 8.8 (High) (GitHub Advisory, Openwall OSS-Sec).

The vulnerability is classified as CWE-415 (Double Free) and resides in Apache's mod_http2 module. When an HTTP/2 connection is reset early by a client (e.g., via a RST_STREAM frame), the server's HTTP/2 handler incorrectly frees the same memory region twice, leading to heap memory corruption. This race condition can be triggered by rapid HTTP/2 frame sequencing, as demonstrated by a public PoC that causes the Apache child process to terminate with a segmentation fault (signal 11). Exploitation requires low privileges (an authenticated or low-barrier network connection) and no user interaction (Openwall OSS-Sec, GitHub PoC, Striga.ai Research). The vulnerability was discovered by Bartlomiej Dmitruk (striga.ai) and Stanislaw Strzalkowski (isec.pl) (Openwall OSS-Sec).

Successful exploitation can result in memory corruption that may allow an authenticated remote attacker to execute arbitrary code with the privileges of the Apache web server process, leading to full system compromise. The vulnerability has high impact on confidentiality, integrity, and availability. At minimum, the double-free condition causes a denial of service by crashing the Apache child process; in a worst-case scenario, it enables arbitrary code execution, potentially allowing lateral movement within the affected infrastructure (GitHub Advisory, Feedly).

A functional proof-of-concept (PoC) exploit (poc.py) is publicly available on GitHub that successfully triggers the race condition and crashes Apache HTTP Server 2.4.66 via the mod_http2 module (GitHub PoC). A non-destructive scanner tool also exists for detection purposes (GitHub Scanner). Exploitation has been reported in the wild according to multiple sources including indusface.com. An Exploit-DB entry (EDB-ID:52577) was published on May 27, 2026 (Exploit-DB). The EPSS score is approximately 0.061% per Feedly data, though the GitHub Advisory Database lists it at 1.123% (79th percentile). The vulnerability is not currently listed in the CISA KEV catalog based on available data.

1. Reconnaissance: Identify Apache HTTP Server instances running version 2.4.66 with HTTP/2 enabled using tools like Shodan, Censys, or the public scanner tool (cve_23918_elite.py) that performs ALPN protocol negotiation analysis and server header inspection.
2. Establish HTTP/2 connection: Initiate an HTTP/2 connection to the target server. Low-privilege credentials or an anonymous connection may suffice depending on server configuration.
3. Trigger early reset race condition: Using the PoC script (poc.py), send rapid HTTP/2 frame sequences designed to trigger an early RST_STREAM reset, causing the mod_http2 handler to free the same memory region twice.
4. Achieve memory corruption: The double-free condition corrupts heap memory in the Apache child process. At minimum, this causes a segmentation fault (DoS); under favorable heap layout conditions, it may allow control of execution flow.
5. Achieve code execution (if conditions permit): With a weaponized payload exploiting the corrupted heap state, execute arbitrary code with the privileges of the Apache web server process (e.g., www-data or apache), enabling reverse shell, data exfiltration, or further lateral movement (GitHub PoC, Openwall OSS-Sec).

• Network: Unusual HTTP/2 traffic patterns with rapid RST_STREAM frames or abnormal connection resets to Apache servers; ALPN negotiation probes from unexpected sources targeting port 443 or 80.
• Logs: Apache error_log entries showing child process crashes with segmentation faults (signal 11); repeated abrupt HTTP/2 connection terminations in access_log from the same source IP; entries referencing mod_http2 errors around connection reset events.
• Process: Apache child processes (httpd) terminating unexpectedly with signal 11 (SIGSEGV); unusual child processes spawned by the Apache parent process.
• File System: Presence of PoC scripts (poc.py, cve_23918_elite.py) or related tooling on attacker-controlled systems; unexpected web shells or new files in the Apache document root or configuration directories following exploitation.
• Detection Rules: Sigma/YARA detection rules available at GitHub Detections; Datadog published HTTP/2 abuse detection guidance for Apache web server logs (Datadog Blog).

The primary remediation is to upgrade Apache HTTP Server from version 2.4.66 to version 2.4.67 or later, which contains the fix (GitHub Advisory, Apache Vulnerabilities). As a temporary workaround if immediate patching is not possible, disable HTTP/2 protocol support in the Apache configuration by removing or commenting out Protocols h2 h2c directives and restarting the service. Distribution-specific patches have been released for Ubuntu (USN-8239-1), Debian (DSA-6248-1, included in Debian 12.14 and 13.5), Slackware (SSA-2026-124-01), Mageia (MGASA-2026-0129), SUSE (SUSE-SU-2026:2104-1), Amazon Linux 2023 (ALAS2023-2026-1724), and openSUSE. Oracle also addressed this in its May 2026 Critical Patch Update (Ubuntu Advisory, Oracle CSP).

The vulnerability received significant coverage across security media and social platforms shortly after disclosure on May 4, 2026. The Hacker News, Security Affairs, SecurityWeek, SC World, and Heise (German/English) all published articles highlighting the critical nature of the flaw and urging immediate patching (The Hacker News, Security Affairs, SecurityWeek). Reddit communities including r/sysadmin, r/webdev, r/cybersecurity, and r/security saw active discussion, with administrators sharing urgency around updating from 2.4.66. The CIS, Belgium CCB, NHS Digital, and Western Australia SOC all issued advisories. Researchers at striga.ai published a technical deep-dive, and the Qualys Threat Protection team published a detailed analysis (Qualys, Striga.ai).