CVE-2026-33007: Apache HTTP Server DOS Vulnerability

CVE-2026-33007 Overview

CVE-2026-33007 is a NULL pointer dereference vulnerability in the mod_authn_socache module of Apache HTTP Server version 2.4.66 and earlier. An unauthenticated remote attacker can crash a child process when the server is deployed in a caching forward proxy configuration. The flaw is tracked under CWE-476 and affects availability without compromising confidentiality or integrity. Apache has released version 2.4.67 to remediate the issue.

Critical Impact

Remote unauthenticated attackers can trigger child process crashes in vulnerable Apache HTTP Server deployments configured as caching forward proxies, resulting in service disruption.

Affected Products

• Apache HTTP Server 2.4.66 and earlier (2.4.x branch)
• Deployments using mod_authn_socache in a caching forward proxy configuration
• Linux, Windows, and Unix builds shipping the affected mod_authn_socache module

Discovery Timeline

• 2026-05-04 - CVE-2026-33007 published to NVD
• 2026-05-04 - Last updated in NVD database

Technical Details for CVE-2026-33007

Vulnerability Analysis

The vulnerability resides in mod_authn_socache, the Apache module that caches authentication credentials in a shared object cache. When Apache HTTP Server is deployed as a caching forward proxy, request handling paths can reach mod_authn_socache code with a pointer that has not been initialized. Dereferencing this pointer causes the worker child process to crash.

The issue is classified under [CWE-476: NULL Pointer Dereference]. Each crash terminates the child process handling the request, forcing the parent process to spawn a replacement. Repeated requests amplify the impact and degrade overall service availability.

The attack does not require authentication, user interaction, or elevated privileges. The attacker only needs network reachability to the affected Apache instance. Confidentiality and integrity are not affected, but availability is degraded as crashes accumulate.

Root Cause

The root cause is missing validation of a pointer used within mod_authn_socache request processing under specific caching forward proxy configurations. The code path executes a dereference operation without first verifying that the pointer references a valid object, leading to a segmentation fault in the child process.

Attack Vector

Exploitation occurs over the network against an Apache HTTP Server configured as a caching forward proxy with mod_authn_socache enabled. An attacker sends crafted HTTP requests through the proxy that trigger the vulnerable code path in the authentication socache module. Each successful request crashes the child process serving the connection.

The vulnerability manifests only when the server runs in a caching forward proxy role with mod_authn_socache loaded. Standard reverse proxy or origin server deployments without this configuration are not affected. See the Apache HTTP Server Vulnerabilities advisory and the Openwall OSS-Security mailing list post for additional technical detail.

Detection Methods for CVE-2026-33007

Indicators of Compromise

• Repeated child process segmentation faults logged in Apache error_log referencing mod_authn_socache
• Unusual frequency of child pid XXXX exit signal Segmentation fault (11) messages
• Spikes in worker process restarts coinciding with anomalous forward proxy request patterns
• Anomalous request bursts targeting authenticated proxy endpoints from a single source

Detection Strategies

• Monitor Apache error_log for segmentation fault messages tied to authentication module code paths
• Inspect process accounting data for elevated worker churn rates on Apache instances acting as forward proxies
• Correlate HTTP access log spikes with child process crash events to identify probing activity
• Inventory Apache instances and verify whether mod_authn_socache is loaded alongside forward proxy directives

Monitoring Recommendations

• Alert on Apache child process crash thresholds exceeding baseline within short windows
• Track Apache version reporting via Server header or configuration management to flag instances running 2.4.66 or earlier
• Forward Apache error and access logs to a centralized analytics platform for correlation across the proxy fleet

How to Mitigate CVE-2026-33007

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.67 or later, as recommended in the Apache HTTP Server Vulnerabilities advisory
• Identify all Apache instances configured as caching forward proxies and prioritize them for patching
• Restrict network exposure of forward proxy endpoints to authorized clients using firewall rules or access controls
• Review Apache configurations to confirm whether mod_authn_socache is required for the deployment

Patch Information

Apache HTTP Server 2.4.67 contains the fix for CVE-2026-33007. Administrators should obtain the updated package from their distribution vendor or directly from the Apache Software Foundation. Verify the upgrade by checking the version reported by httpd -v after deployment and reload the service for the changes to take effect.

Workarounds

• Disable mod_authn_socache by commenting out the corresponding LoadModule authn_socache_module directive if it is not required
• Remove caching forward proxy configuration where the role is not needed in production
• Place a hardened upstream proxy or web application firewall in front of affected Apache instances to filter malformed requests until patching is complete

# Configuration example: disable mod_authn_socache until upgrade
# Edit httpd.conf or the relevant modules configuration file
# Comment out the following line:
# LoadModule authn_socache_module modules/mod_authn_socache.so

# Verify Apache version after upgrade
httpd -v

# Restart Apache to apply configuration changes
sudo systemctl restart httpd