CVE-2026-23918: Apache HTTP Server RCE Vulnerability

CVE-2026-23918 Overview

CVE-2026-23918 is a double free vulnerability [CWE-415] in the Apache HTTP Server HTTP/2 protocol implementation. The flaw affects Apache HTTP Server version 2.4.66 and may permit remote code execution. The Apache HTTP Server Project recommends upgrading to version 2.4.67, which contains the fix. A public proof-of-concept is available, increasing the urgency of remediation for internet-facing web servers running the affected release.

Critical Impact

An authenticated network attacker can trigger a double free in the HTTP/2 handler, potentially leading to remote code execution and full compromise of the web server process.

Affected Products

• Apache HTTP Server 2.4.66 with HTTP/2 (mod_http2) enabled
• Deployments terminating HTTP/2 traffic at the Apache web server
• Reverse proxy and origin server configurations using Apache 2.4.66

Discovery Timeline

• 2026-05-04 - CVE-2026-23918 published to the National Vulnerability Database
• 2026-05-04 - Apache HTTP Server Project publishes vendor advisory and version 2.4.67
• 2026-05-04 - Last updated in the NVD database

Technical Details for CVE-2026-23918

Vulnerability Analysis

The vulnerability is a double free condition in the Apache HTTP Server HTTP/2 protocol stack. A double free occurs when the same memory region is released twice through free() or an equivalent allocator call. This corrupts allocator metadata and can be weaponized for arbitrary write primitives.

In the context of mod_http2, processing of crafted HTTP/2 frames or stream state transitions causes a memory region tied to a request, stream, or connection structure to be released twice. The resulting heap corruption can be steered toward control flow hijack, enabling remote code execution in the worker process. The attack is performed over the network with low complexity, though the issue requires some form of privilege according to the CVSS vector.

Successful exploitation grants the attacker the privileges of the Apache worker process. This typically allows access to served content, configuration files, TLS keys, and any backend systems the web server can reach.

Root Cause

The root cause is incorrect lifetime management of a heap-allocated object within the HTTP/2 handler. A code path frees an object that has already been released, violating ownership invariants. CWE-415 describes this class of bug, which is common when reference counting, error handling, or stream cleanup logic intersect.

Attack Vector

Exploitation requires network access to an HTTP/2 endpoint served by Apache HTTP Server 2.4.66. The attacker sends a sequence of HTTP/2 frames that drives the vulnerable cleanup path. A public proof-of-concept exists at the CVE-2026-23918 Apache H2 PoC repository, which lowers the barrier for exploitation.

No verified exploitation code is included here. Refer to the Apache HTTP Server Vulnerabilities advisory and the Openwall OSS Security discussion for technical detail.

Detection Methods for CVE-2026-23918

Indicators of Compromise

• Apache worker process crashes (segfault, SIGABRT) recorded in error_log or systemd journal during HTTP/2 traffic
• Glibc allocator messages such as double free or corruption in process output or core dumps
• Unexpected child processes spawned by httpd or apache2 user
• Outbound connections from the web server process to unfamiliar hosts following malformed HTTP/2 sessions

Detection Strategies

• Inventory all Apache HTTP Server instances and flag any running version 2.4.66 with mod_http2 loaded
• Inspect HTTP/2 traffic for anomalous frame sequences, premature stream resets, and abnormal GOAWAY patterns
• Correlate web server crashes and restarts with inbound HTTP/2 client IPs in access logs
• Hunt for post-exploitation behavior such as shell spawning from httpd parent processes

Monitoring Recommendations

• Enable core dump collection on Apache hosts to capture evidence of heap corruption
• Forward error_log, access_log, and auditd events to a centralized analytics platform for correlation
• Alert on any execution of /bin/sh, bash, or scripting interpreters as a child of the Apache worker
• Track external network connections initiated by the Apache process against an established baseline

How to Mitigate CVE-2026-23918

Immediate Actions Required

• Upgrade Apache HTTP Server from 2.4.66 to 2.4.67 or later on all affected hosts
• Restart the httpd or apache2 service after patching to ensure the vulnerable code is unloaded
• Audit reverse proxies and load balancers to confirm the patched version is serving HTTP/2
• Review web server logs for crashes or anomalies that occurred prior to patching

Patch Information

The Apache HTTP Server Project addresses CVE-2026-23918 in version 2.4.67. Source tarballs and binary builds are available from the project. Refer to the Apache HTTP Server Vulnerabilities advisory for the official fix metadata and changelog entry.

Workarounds

• Disable HTTP/2 by removing Protocols h2 h2c directives and unloading mod_http2 until the patch is applied
• Terminate HTTP/2 at an upstream proxy or CDN that is not running Apache 2.4.66 and forward HTTP/1.1 to the origin
• Restrict access to HTTP/2 endpoints with network ACLs where the service is not internet-facing

# Disable mod_http2 on Debian/Ubuntu until patch is applied
sudo a2dismod http2
sudo systemctl restart apache2

# Verify installed Apache version after upgrade
httpd -v   # RHEL/CentOS
apache2 -v # Debian/Ubuntu
# Expected: Server version: Apache/2.4.67 or later