CVE-2026-23918: Apache HTTP Server Double Free / RCE via HTTP/2

CVE-2026-23918: Apache HTTP Server HTTP/2 Double Free

The Apache HTTP Server Project has patched a high-severity double-free vulnerability tracked as CVE-2026-23918, carrying a CVSS score of 8.8. The flaw exists in the HTTP/2 protocol handling code within Apache HTTP Server version 2.4.66 and can be leveraged by a remote attacker to corrupt heap memory, potentially leading to arbitrary code execution on the server.

All systems running Apache HTTP Server 2.4.66 should upgrade to 2.4.67 immediately, which contains the fix.

Vulnerability Details

Technical Analysis

Root Cause

A double-free memory corruption occurs within Apache's HTTP/2 module (mod_http2) during connection teardown or request processing under specific error conditions. When a specially crafted HTTP/2 request triggers an error path, a pointer to a heap-allocated memory region is freed twice. On modern allocators, a double-free can cause:

• Heap metadata corruption — overwriting allocator bookkeeping structures
• Use-after-free conditions — subsequent allocations landing in the freed region
• Remote code execution — through controlled heap layout manipulation

Because HTTP/2 support is commonly enabled on production Apache deployments and the attack requires no authentication, the risk surface is broad.

Attack Surface

1. Attacker sends a specially crafted HTTP/2 request to any Apache 2.4.66 endpoint
   with HTTP/2 enabled (h2 or h2c)
 
2. Malformed request triggers an error condition in mod_http2
   that invokes free() on an already-freed pointer
 
3. Double-free corrupts heap metadata or reallocates the freed region
   to attacker-controlled content
 
4. Attacker achieves arbitrary code execution as the Apache worker process user
   (typically www-data, apache, or httpd)

Why CVSS 8.8

The score falls below 9.0 primarily due to the practical complexity of reliable heap exploitation in modern environments with ASLR and allocator hardening, though these are mitigations rather than eliminations of the risk.

Affected Deployments

Apache HTTP Server is one of the most widely deployed web servers on the internet, with tens of millions of instances. Any organization running version 2.4.66 should treat this as an urgent patching priority.

Remediation

Step 1: Upgrade to 2.4.67

The definitive fix is upgrading to Apache HTTP Server 2.4.67, which corrects the double-free in mod_http2.

# Debian / Ubuntu
sudo apt-get update && sudo apt-get upgrade apache2
 
# RHEL / CentOS / Fedora
sudo dnf update httpd
 
# Verify installed version
apache2 -v   # or httpd -v

Confirm the version reports 2.4.67 or higher after the upgrade.

Step 2: Interim Mitigation — Disable HTTP/2

If an immediate upgrade is not operationally possible, disable HTTP/2 as a temporary measure:

# In httpd.conf or ssl.conf
# Remove or comment out H2 and h2c protocol support
Protocols h1.1
# or explicitly:
# H2Direct off

Note: Disabling HTTP/2 degrades performance for HTTP/2-capable clients but eliminates the attack surface for this specific vulnerability. Re-enable after patching.

Step 3: Verify Module Load

Confirm whether mod_http2 is loaded in your deployment:

apache2ctl -M | grep http2
# or
httpd -M | grep http2

If http2_module appears in the output and you are running 2.4.66, you are vulnerable until patched or HTTP/2 is disabled.

Detection

Check for Exploitation Indicators

Review Apache error logs for anomalous memory-related messages or crashes:

# Look for double-free or heap corruption signals
grep -E "double free|heap corruption|segfault|SIGSEGV|mod_http2" /var/log/apache2/error.log
 
# Check for core dumps
ls -la /var/crash/ /tmp/*.core 2>/dev/null

Web Application Firewall Rule

Block malformed HTTP/2 HEADERS frames with unusual padding or field count values. Most enterprise WAFs (AWS WAF, Cloudflare, ModSecurity) can be configured to drop malformed HTTP/2 frames — consult vendor guidance for specific rule IDs.

Key Takeaways

1. CVE-2026-23918 is a CVSS 8.8 High double-free memory corruption in Apache HTTP Server 2.4.66's HTTP/2 module
2. Successful exploitation can lead to remote code execution as the Apache worker process user
3. The vulnerability requires no authentication and is exploitable over the network
4. Upgrade to Apache HTTP Server 2.4.67 is the only complete fix
5. As an interim measure, disable HTTP/2 (mod_http2) until patching is possible

Sources

• NVD — CVE-2026-23918
• Apache HTTP Server Security Advisories