CVE-2023-31122: Apache HTTP Server Out-of-Bounds Read

CVE-2023-31122 Overview

CVE-2023-31122 is an Out-of-bounds Read vulnerability affecting the mod_macro module of Apache HTTP Server. This vulnerability allows remote attackers to trigger out-of-bounds memory read operations, potentially leading to denial of service conditions. The flaw impacts Apache HTTP Server versions through 2.4.57 and has been addressed in subsequent releases.

Critical Impact

This vulnerability can be exploited remotely without authentication to cause denial of service by triggering out-of-bounds memory reads in the mod_macro module, potentially crashing affected Apache HTTP Server instances.

Affected Products

• Apache HTTP Server through version 2.4.57
• Debian Linux 10.0
• Fedora 38

Discovery Timeline

• 2023-10-23 - CVE-2023-31122 published to NVD
• 2025-08-01 - Last updated in NVD database

Technical Details for CVE-2023-31122

Vulnerability Analysis

This vulnerability exists in the mod_macro module of Apache HTTP Server, which provides macro functionality for configuration files. The out-of-bounds read condition (CWE-125) occurs when the module improperly handles certain inputs during macro processing, allowing memory to be read beyond allocated buffer boundaries.

When exploited, an attacker can cause the server to read memory outside of intended boundaries, which can lead to information exposure or, more critically in this case, denial of service through server crashes. The vulnerability can be triggered remotely over the network without requiring any authentication or user interaction, making it particularly concerning for internet-facing Apache installations.

The impact primarily affects availability, as successful exploitation results in server crashes or service disruption rather than data theft or system compromise.

Root Cause

The root cause of CVE-2023-31122 lies in insufficient bounds checking within the mod_macro module when processing macro definitions or expansions. The module fails to properly validate input lengths or array indices before performing memory read operations, allowing attackers to craft requests that trigger reads beyond allocated buffer boundaries.

Attack Vector

The vulnerability is exploitable remotely via the network. An attacker can send specially crafted HTTP requests to a vulnerable Apache HTTP Server instance that has the mod_macro module enabled. The attack does not require authentication or any user interaction, making it straightforward to exploit.

The attack flow typically involves:

1. Identifying a target Apache HTTP Server with mod_macro enabled
2. Crafting malicious requests designed to trigger the out-of-bounds read condition
3. Sending the requests to cause memory access violations
4. The server process crashes or becomes unresponsive, resulting in denial of service

Since mod_macro is not enabled by default in most Apache installations, the attack surface is limited to servers that have explicitly enabled this module.

Detection Methods for CVE-2023-31122

Indicators of Compromise

• Unexpected Apache HTTP Server crashes or segmentation faults in server logs
• Error messages referencing mod_macro module failures or memory access violations
• Increased server restarts or watchdog process activations
• Log entries showing unusual macro-related processing errors

Detection Strategies

• Monitor Apache error logs for segmentation faults or memory-related errors involving mod_macro
• Implement application-level monitoring to detect unexpected Apache process terminations
• Deploy intrusion detection rules to identify anomalous HTTP request patterns targeting macro functionality
• Review server configurations to identify systems with mod_macro enabled

Monitoring Recommendations

• Enable detailed Apache error logging to capture crash diagnostics
• Configure process monitoring to alert on unexpected Apache worker or parent process terminations
• Implement network traffic analysis to detect potential exploitation attempts
• Set up availability monitoring to quickly identify service disruptions

How to Mitigate CVE-2023-31122

Immediate Actions Required

• Update Apache HTTP Server to version 2.4.58 or later, which contains the security fix
• If immediate patching is not possible, disable the mod_macro module until the update can be applied
• Review server configurations to identify all instances using mod_macro
• Monitor server logs for signs of exploitation attempts

Patch Information

Apache has released a security patch addressing this vulnerability in Apache HTTP Server version 2.4.58. Organizations should update to this version or later to remediate the vulnerability. Security advisories and patch information are available from the Apache HTTP Server Security Vulnerabilities page.

Additional patch information is available from distribution-specific advisories:

• Debian LTS Security Announcement
• Fedora Package Announcements
• NetApp Security Advisory

Workarounds

• Disable mod_macro module if not required by removing or commenting out the LoadModule macro_module directive
• Restrict access to the Apache server using firewall rules to limit exposure to trusted networks only
• Implement a web application firewall (WAF) to filter potentially malicious requests
• Consider using reverse proxy configurations to add an additional layer of protection

# Configuration example - Disable mod_macro in Apache
# Comment out or remove the following line in httpd.conf or modules configuration:
# LoadModule macro_module modules/mod_macro.so

# Verify mod_macro is disabled by checking loaded modules:
apachectl -M | grep macro

# Restart Apache after configuration changes:
systemctl restart httpd
# or
systemctl restart apache2