If you are running Apache HTTP Server version 2.4.66, you need to act now. A double-free memory corruption vulnerability — CVE-2026-23918, rated CVSS 8.8 (High) — in Apache’s mod_http2 module can be exploited by a remote attacker for Denial of Service and, under common configurations, for full Remote Code Execution (RCE).
What Is the Vulnerability?
CVE-2026-23918 is a double-free bug inside h2_mplx.c, the stream multiplexer in Apache’s HTTP/2 module. The flaw lives in the stream cleanup code path and is triggered by a specific sequence of HTTP/2 frames:
- The client sends an HTTP/2 HEADERS frame to initiate a request.
- Before the stream is fully registered internally, the client immediately sends a RST_STREAM frame with a non-zero error code.
This race-condition-like sequence causes Apache to attempt to free the same memory region twice — a classic double-free that corrupts the process heap. In the best case for the attacker, this crashes the Apache worker process (DoS). In the worst case, it leads to arbitrary code execution.
Who Is Affected?
This vulnerability is exclusively in Apache httpd 2.4.66. All other versions of Apache are unaffected. If you are running 2.4.65 or earlier, or if you have already upgraded to 2.4.67, you are safe.
However, within that single affected version:
- Denial of Service: Any deployment with mod_http2 enabled is trivially vulnerable. An attacker with internet access can reliably crash Apache worker processes.
- Remote Code Execution: RCE is achievable when Apache is compiled against APR (Apache Portable Runtime) with the mmap allocator. This is the default configuration on Debian-based systems (including Ubuntu) and in the official Apache Docker image. If you are running Apache 2.4.66 in a container or on a Debian/Ubuntu host, assume you are in the RCE risk category.
How Bad Is Remote Code Execution Here?
Full server compromise. An attacker exploiting the RCE path can execute arbitrary code with the privileges of the Apache process (commonly www-data on Debian systems). From there, they can:
- Read application source code, configuration files, and database credentials
- Plant webshells for persistent backdoor access
- Pivot to internal network resources
- Escalate privileges using a secondary local exploit
Because HTTP/2 is enabled by default in most modern Apache configurations, no unusual setup is required on the victim’s side.
How to Fix It
Primary fix: upgrade to Apache 2.4.67, which contains the corrected stream cleanup logic. This is the only complete remediation.
If an immediate upgrade is not possible, apply this interim mitigation in your Apache configuration:
- Disable HTTP/2 by removing or commenting out the
Protocols h2 http/1.1directive (or explicitly settingProtocols http/1.1). This eliminates the attack surface entirely until the upgrade can be performed. Note that this will reduce performance for clients that support HTTP/2.
Check your Apache version:
apache2 -v(Debian/Ubuntu)httpd -v(RHEL/CentOS)
If the output shows 2.4.66, treat this as an emergency and act immediately.
Narrow Scope, High Severity
The narrow version scope (only 2.4.66) is fortunate, but it does not reduce urgency for those who are affected. Apache 2.4.66 was released as a recommended upgrade and saw significant adoption. If your infrastructure uses automated dependency management or Docker image pulls, you may have picked it up without realizing it. Audit your servers and containers now.
At Solvanta, we help businesses keep their infrastructure protected and secure — including web server hardening, vulnerability scanning, and rapid patch response. If you’re unsure whether your Apache deployment is at risk or need help managing your upgrade process, contact our team.