CVE-2024-38473: Apache HTTP Server Auth Bypass Flaw

CVE-2024-38473 Overview

An encoding problem vulnerability exists in the mod_proxy module of Apache HTTP Server versions 2.4.59 and earlier. This flaw allows request URLs with incorrect encoding to be forwarded to backend services, potentially enabling attackers to bypass authentication mechanisms through specially crafted requests. The vulnerability stems from improper output encoding or escaping (CWE-116), which can lead to unauthorized access to protected resources on backend servers.

Critical Impact

Attackers can exploit this encoding vulnerability to bypass authentication on backend services, potentially gaining unauthorized access to sensitive data or functionality. The vulnerability affects a widely deployed web server component, increasing the scope of potential impact.

Affected Products

• Apache HTTP Server versions up to and including 2.4.59
• NetApp ONTAP 9
• Systems using mod_proxy for reverse proxy configurations

Discovery Timeline

• 2024-07-01 - CVE-2024-38473 published to NVD
• 2025-07-01 - Last updated in NVD database

Technical Details for CVE-2024-38473

Vulnerability Analysis

This vulnerability affects the mod_proxy module, which is a core component used for proxying HTTP requests in Apache HTTP Server deployments. When mod_proxy processes incoming requests, it fails to properly validate and normalize URL encoding before forwarding requests to backend services. This improper output encoding allows malicious actors to craft requests that appear legitimate to the proxy but are interpreted differently by backend servers, potentially bypassing authentication controls that rely on URL-based access restrictions.

The attack is network-accessible and requires low privileges to execute, making it relatively easy to exploit in environments where Apache is configured as a reverse proxy in front of authenticated backend services.

Root Cause

The root cause is classified under CWE-116 (Improper Encoding or Escaping of Output). The mod_proxy module does not adequately sanitize or normalize URL-encoded characters in request URLs before forwarding them to backend services. This encoding inconsistency creates a disparity between how the proxy interprets the URL path and how the backend service processes it, enabling authentication bypass scenarios.

Attack Vector

The attack is conducted over the network by sending specially crafted HTTP requests through the Apache proxy. An attacker can manipulate URL encoding in requests to bypass path-based authentication rules. For example, if a backend service expects authentication for certain URL paths, the attacker may use alternative encoding representations that the proxy passes through unchanged but are decoded differently by the backend, granting unauthorized access.

The attack flow typically involves:

1. Attacker identifies an Apache server configured with mod_proxy
2. Attacker crafts requests with manipulated URL encoding targeting protected backend paths
3. The mod_proxy module forwards the request without proper encoding normalization
4. The backend service interprets the malformed encoding differently, potentially bypassing authentication checks

Detection Methods for CVE-2024-38473

Indicators of Compromise

• Unusual URL-encoded sequences in Apache access logs, particularly double-encoding or mixed encoding patterns
• Unexpected access to protected backend resources without corresponding authentication events
• HTTP requests containing malformed or non-standard percent-encoded characters targeting proxy endpoints
• Authentication bypass attempts visible in backend application logs

Detection Strategies

• Monitor Apache access logs for requests containing suspicious URL encoding patterns such as %2e, %2f, or unusual character sequences
• Implement Web Application Firewall (WAF) rules to detect and block requests with abnormal URL encoding
• Correlate proxy access logs with backend authentication logs to identify requests that bypassed expected authentication
• Deploy intrusion detection signatures targeting mod_proxy encoding bypass attempts

Monitoring Recommendations

• Enable detailed logging for mod_proxy including request URLs and headers
• Configure alerting for high volumes of 4xx/5xx responses from backend services that may indicate exploitation attempts
• Implement log analysis to detect patterns of encoded path traversal or authentication bypass attempts
• Monitor for unauthorized access patterns on backend services that are fronted by Apache reverse proxies

How to Mitigate CVE-2024-38473

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.60 or later immediately
• Review mod_proxy configurations and restrict proxy access to only necessary backend services
• Implement additional authentication at the backend service level rather than relying solely on proxy-based access controls
• Deploy WAF rules to block requests with suspicious URL encoding patterns

Patch Information

Apache has released version 2.4.60 which addresses this encoding vulnerability in mod_proxy. Organizations should prioritize upgrading to this version. For detailed information about the fix and other security updates, refer to the Apache HTTP Server Vulnerabilities page. NetApp customers should also review the NetApp Security Advisory NTAP-20240712-0001 for ONTAP-specific guidance.

Workarounds

• Implement strict input validation at the backend service level to reject improperly encoded requests
• Configure mod_security or similar WAF solutions to normalize URL encoding before requests reach backend services
• Use ProxyPass directives with explicit path restrictions to limit proxy scope
• Consider implementing additional authentication layers that do not rely solely on URL path matching

# Configuration example - Additional mod_security rules for URL encoding validation
# Add to Apache configuration or mod_security rules file

# Enable URL normalization
SecRule REQUEST_URI "@validateUrlEncoding" "id:1001,phase:1,deny,status:400,msg:'Invalid URL Encoding Detected'"

# Block double-encoded requests
SecRule REQUEST_URI "@contains %25" "id:1002,phase:1,deny,status:400,msg:'Double Encoding Attempt Blocked'"

# Restrict proxy to specific backend paths only
<Location /api>
    ProxyPass http://backend-server:8080/api
    ProxyPassReverse http://backend-server:8080/api
    # Require explicit authentication at proxy level
    AuthType Basic
    AuthName "API Access"
    Require valid-user
</Location>