CVE-2024-39573: Apache HTTP Server SSRF Vulnerability

CVE-2024-39573 Overview

CVE-2024-39573 is a Server-Side Request Forgery (SSRF) vulnerability in the mod_rewrite module of Apache HTTP Server versions 2.4.59 and earlier. This vulnerability allows an attacker to exploit unsafe RewriteRules configurations to unexpectedly route URL requests through mod_proxy, potentially enabling unauthorized access to internal resources or backend services.

The flaw stems from improper input validation in how mod_rewrite processes certain URL patterns. When specific RewriteRules are configured, an attacker can craft malicious requests that cause the server to proxy connections to unintended destinations, effectively turning the Apache server into a pivot point for internal network reconnaissance or attacks against backend infrastructure.

Critical Impact

This SSRF vulnerability can allow attackers to bypass network security controls, access internal services, scan internal networks, and potentially exfiltrate sensitive data from protected resources behind the web server.

Affected Products

• Apache HTTP Server versions up to and including 2.4.59
• NetApp ONTAP 9

Discovery Timeline

• 2024-07-01 - CVE-2024-39573 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-39573

Vulnerability Analysis

The vulnerability resides in the interaction between Apache's mod_rewrite and mod_proxy modules. When administrators configure RewriteRules to handle URL transformations, certain rule patterns can inadvertently enable proxy behavior that was not intended by the configuration author.

The core issue involves improper handling of URL schemes and destinations within rewrite operations. Under specific conditions, mod_rewrite can transform a request in a way that triggers mod_proxy to forward the request to an arbitrary destination controlled by the attacker. This behavior bypasses the typical access controls that administrators expect to govern proxy functionality.

The attack is particularly dangerous because it can occur even when administrators believe they have not explicitly enabled proxy functionality for certain URL patterns. The vulnerability effectively creates a "confused deputy" scenario where the web server acts on behalf of the attacker against internal resources.

Root Cause

The root cause is classified as CWE-20 (Improper Input Validation). The vulnerability occurs because mod_rewrite does not adequately validate or sanitize URL components before passing them to mod_proxy for handling. This allows specially crafted input to escape the intended rewrite context and establish proxy connections to attacker-specified destinations.

Specifically, the parsing logic in mod_rewrite fails to properly distinguish between legitimate URL transformations and those that would result in proxy requests to unauthorized destinations. This insufficient validation creates a gap between the administrator's intended security policy and the actual behavior of the server.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending crafted HTTP requests to an Apache server with vulnerable RewriteRules configurations.

The exploitation flow involves:

1. Attacker identifies an Apache HTTP Server with mod_rewrite enabled and potentially vulnerable rule configurations
2. Attacker crafts HTTP requests with specially designed URLs or parameters
3. The mod_rewrite module processes these requests and, due to the validation flaw, sets up the URL to be handled by mod_proxy
4. The Apache server makes outbound connections to the attacker-specified destination, which could be internal services, cloud metadata endpoints, or other protected resources
5. Responses from these internal requests may be returned to the attacker, enabling data exfiltration or further reconnaissance

For detailed technical information about the vulnerability mechanism, see the Apache HTTPD Security Advisory and the discussion on the Openwall OSS-Security mailing list.

Detection Methods for CVE-2024-39573

Indicators of Compromise

• Unusual outbound connections from the Apache web server to internal IP ranges or unexpected external destinations
• HTTP access logs showing requests with unusual URL patterns designed to trigger proxy behavior
• Error logs containing unexpected proxy-related errors or connection attempts
• Network traffic from the web server to internal services that should not be accessible via the web tier
• Requests attempting to access cloud metadata endpoints (e.g., 169.254.169.254) through the web server

Detection Strategies

• Implement network monitoring to detect outbound connections from web servers to internal resources or metadata services
• Configure web application firewalls (WAF) to inspect and block requests with SSRF-indicative patterns
• Deploy intrusion detection systems (IDS) with rules specifically designed to detect SSRF attempts against Apache servers
• Establish baseline network behavior for Apache servers and alert on anomalous outbound connection patterns

Monitoring Recommendations

• Enable detailed Apache access logging and regularly analyze logs for suspicious URL patterns
• Monitor network flows from Apache server instances to detect unexpected internal communications
• Implement DNS query logging to identify attempts to resolve internal hostnames through SSRF exploitation
• Set up alerts for requests containing common SSRF payloads such as internal IP addresses, localhost references, or cloud metadata URLs

How to Mitigate CVE-2024-39573

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.60 or later, which contains the fix for this vulnerability
• Audit existing RewriteRules configurations to identify and remediate potentially unsafe patterns
• If immediate patching is not possible, review and restrict mod_proxy configurations to minimize exposure
• Implement network segmentation to limit the potential impact of SSRF attacks from web servers

Patch Information

Apache has released version 2.4.60 which addresses this SSRF vulnerability. Users running Apache HTTP Server versions 2.4.59 and earlier should upgrade immediately to the patched version.

Detailed patch information and security advisories are available at the Apache HTTPD Vulnerabilities List. For NetApp ONTAP deployments, consult the NetApp Security Advisory NTAP-20240712-0001 for product-specific guidance.

Workarounds

• Disable mod_proxy if it is not required for your deployment to eliminate the attack surface
• Implement strict allowlisting in mod_proxy configuration to restrict destinations to known, trusted endpoints only
• Use the ProxyRequests Off directive to disable forward proxy functionality if not needed
• Configure network-level controls (firewalls, security groups) to restrict outbound connections from Apache servers to only necessary destinations

# Configuration example - Restrict mod_proxy to prevent SSRF exploitation
# Add to Apache configuration or .htaccess

# Disable forward proxy requests
ProxyRequests Off

# If proxy is required, implement strict allowlist
<Proxy *>
    Require all denied
</Proxy>

# Only allow proxying to specific trusted backends
<ProxyMatch "^https://trusted-backend\.example\.com">
    Require all granted
</ProxyMatch>