CVE-2024-40898: Apache HTTP Server SSRF Vulnerability

CVE-2024-40898 Overview

CVE-2024-40898 is a Server-Side Request Forgery (SSRF) vulnerability affecting Apache HTTP Server on Windows systems when mod_rewrite is configured in server or vhost context. This vulnerability allows attackers to potentially leak NTLM hashes to a malicious server through crafted requests, enabling credential theft and potential lateral movement within an organization's network.

Critical Impact

Attackers can exploit this SSRF vulnerability to capture NTLM authentication hashes from Windows servers running Apache HTTP Server, potentially leading to credential theft, pass-the-hash attacks, and unauthorized access to network resources.

Affected Products

• Apache HTTP Server (versions prior to 2.4.62)
• Microsoft Windows (all versions running vulnerable Apache HTTP Server)
• Systems with mod_rewrite enabled in server/vhost context

Discovery Timeline

• 2024-07-18 - CVE-2024-40898 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-40898

Vulnerability Analysis

This vulnerability exploits the interaction between Apache HTTP Server's mod_rewrite module and Windows NTLM authentication mechanisms. When mod_rewrite is configured in a server or virtual host context on Windows systems, specially crafted requests can trigger outbound connections to attacker-controlled servers. During these connections, Windows may automatically attempt NTLM authentication, inadvertently transmitting credential hashes to the malicious endpoint.

The attack is particularly dangerous because it combines web server misconfiguration with Windows' default authentication behavior. The SSRF component allows attackers to redirect internal server requests, while Windows' NTLM auto-authentication means credentials can be leaked without explicit user interaction.

Root Cause

The root cause lies in improper validation of URL destinations within mod_rewrite rules when operating in server/vhost context. The module fails to adequately restrict outbound requests to trusted destinations, allowing attackers to craft requests that force the server to connect to arbitrary external hosts. On Windows systems, this becomes a credential exposure issue because the operating system may automatically attempt NTLM authentication when connecting to SMB or WebDAV resources.

Attack Vector

The attack leverages network-accessible Apache HTTP Server instances with mod_rewrite enabled. An attacker sends malicious requests designed to trigger rewrite rules that result in outbound connections to attacker-controlled infrastructure. The attack requires no authentication and exploits the server's default behavior when processing URL rewrites.

The exploitation flow involves:

1. Attacker identifies a Windows-based Apache HTTP Server with mod_rewrite enabled
2. Crafted HTTP requests are sent to trigger SSRF through rewrite rules
3. The server initiates connections to attacker-specified destinations
4. Windows automatically attempts NTLM authentication to the malicious server
5. NTLM hashes are captured and can be cracked offline or used in relay attacks

Detection Methods for CVE-2024-40898

Indicators of Compromise

• Unusual outbound connections from Apache HTTP Server processes to external IP addresses
• Unexpected SMB or WebDAV traffic originating from web server hosts
• NTLM authentication attempts to untrusted external servers
• Anomalous mod_rewrite activity in Apache access and error logs

Detection Strategies

• Monitor network traffic for outbound connections from web servers to non-standard destinations
• Analyze Apache access logs for suspicious URL patterns that may trigger SSRF
• Implement network segmentation monitoring to detect web servers initiating unexpected outbound connections
• Deploy endpoint detection rules to identify NTLM authentication attempts to external hosts

Monitoring Recommendations

• Configure SIEM alerts for outbound SMB/WebDAV connections from web server hosts
• Enable verbose logging for mod_rewrite to capture rule processing details
• Monitor Windows Security Event logs for NTLM authentication events (Event ID 4624, 4625)
• Implement network-level detection for NTLM traffic leaving trusted network boundaries

How to Mitigate CVE-2024-40898

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.62 or later immediately
• Review and audit all mod_rewrite configurations in server and vhost contexts
• Implement outbound firewall rules to restrict web server egress traffic
• Consider disabling NTLM authentication where possible or implement Extended Protection for Authentication

Patch Information

Apache has released version 2.4.62 which addresses this vulnerability. Users should upgrade immediately by downloading the latest version from the Apache HTTP Server Vulnerabilities page. The patch implements proper validation of rewrite destinations to prevent SSRF attacks.

Additional security advisories have been issued by NetApp Security Advisory NTAP-20240808-0006 for affected products in their portfolio.

Workarounds

• Restrict outbound network access from web servers using host-based firewalls
• Block SMB (TCP 445) and WebDAV traffic from web server hosts at the network perimeter
• Implement URL validation in mod_rewrite rules to allow only trusted destinations
• Consider using mod_security or similar WAF solutions to filter malicious requests

# Configuration example
# Block outbound SMB traffic from Apache server (Windows Firewall)
netsh advfirewall firewall add rule name="Block Outbound SMB" dir=out action=block protocol=tcp remoteport=445

# Restrict mod_rewrite to internal hosts only (Apache configuration)
# Add to httpd.conf or virtual host configuration
RewriteCond %{HTTP_HOST} !^(trusted-host1\.example\.com|trusted-host2\.example\.com)$ [NC]
RewriteRule .* - [F,L]