CVE-2024-47220: WEBrick HTTP Request Smuggling Vulnerability

CVE-2024-47220 Overview

CVE-2024-47220 is an HTTP Request Smuggling vulnerability discovered in the WEBrick toolkit through version 1.8.1 for Ruby. The vulnerability allows attackers to smuggle HTTP requests by providing both a Content-Length header and a Transfer-Encoding header in the same request, enabling malicious requests to be embedded within legitimate ones.

Critical Impact

This vulnerability enables attackers to bypass security controls, poison web caches, and potentially gain unauthorized access to protected resources by embedding hidden requests within legitimate HTTP traffic.

Affected Products

• WEBrick toolkit versions up to and including 1.8.1
• Ruby applications utilizing vulnerable WEBrick versions as a web server

Discovery Timeline

• 2024-09-22 - CVE-2024-47220 published to NVD
• 2025-01-09 - Last updated in NVD database

Technical Details for CVE-2024-47220

Vulnerability Analysis

HTTP Request Smuggling occurs when a web server or proxy does not properly handle ambiguous HTTP requests containing conflicting length indicators. In the case of CVE-2024-47220, WEBrick fails to properly validate and reject requests that include both Content-Length and Transfer-Encoding headers simultaneously.

According to RFC 7230, when both headers are present, compliant HTTP implementations must either reject the request or ignore the Content-Length header entirely. WEBrick's improper handling of this condition creates a desynchronization vulnerability that attackers can exploit to inject malicious requests.

The vendor has noted that "WEBrick should not be used in production," indicating this library is intended for development and testing purposes only. However, applications that inadvertently deploy WEBrick in production environments remain vulnerable to this attack.

Root Cause

The root cause of this vulnerability lies in WEBrick's HTTP request parsing logic, which fails to properly handle the edge case where both Content-Length and Transfer-Encoding headers are present in the same request. This violates HTTP/1.1 specification requirements and creates an ambiguity that attackers can exploit to desynchronize request boundaries between front-end proxies and the back-end WEBrick server.

Attack Vector

An attacker can craft a malicious HTTP request containing both Content-Length and Transfer-Encoding: chunked headers. When this request passes through a reverse proxy or load balancer to a WEBrick backend, the two systems may interpret the request boundaries differently. This allows the attacker to embed a second, hidden request (such as GET /admin HTTP/1.1) within what appears to be a legitimate request (such as POST /user HTTP/1.1).

The smuggled request can then be processed by the backend server in the context of another user's session, potentially leading to unauthorized access to protected resources, cache poisoning, or session hijacking.

Detection Methods for CVE-2024-47220

Indicators of Compromise

• HTTP requests containing both Content-Length and Transfer-Encoding headers simultaneously
• Unusual access patterns to restricted endpoints such as /admin without proper authentication flow
• Log entries showing mismatched request methods or unexpected request sequences
• Evidence of cache poisoning with unauthorized content being served to legitimate users

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block requests containing both Content-Length and Transfer-Encoding headers
• Enable detailed HTTP request logging on both proxy and backend servers to identify request desynchronization
• Monitor for anomalous access patterns to sensitive endpoints that bypass normal authentication workflows
• Deploy intrusion detection system (IDS) signatures specifically targeting HTTP request smuggling patterns

Monitoring Recommendations

• Configure alerts for HTTP requests with conflicting length headers arriving at WEBrick instances
• Establish baseline metrics for request patterns and alert on deviations that may indicate smuggling attempts
• Review web server and proxy logs regularly for signs of request boundary desynchronization
• Monitor for unexpected cache behavior or content served from cache that does not match expected responses

How to Mitigate CVE-2024-47220

Immediate Actions Required

• Upgrade WEBrick to a patched version that addresses this vulnerability - see GitHub Pull Request #146 for the fix
• Replace WEBrick with a production-grade web server such as Puma, Unicorn, or Passenger for production deployments
• Configure front-end proxies and load balancers to reject or normalize requests containing both Content-Length and Transfer-Encoding headers
• Audit existing Ruby applications to identify any unintended use of WEBrick in production environments

Patch Information

A fix for this vulnerability has been committed to the WEBrick repository. The patch is available in GitHub Pull Request #146. Organizations should update to a patched version of WEBrick or migrate to a production-ready web server. For additional context and discussion, refer to GitHub Issue #145.

Workarounds

• Deploy a reverse proxy (such as nginx or HAProxy) in front of WEBrick configured to reject requests with both Content-Length and Transfer-Encoding headers
• Implement request validation at the application layer to detect and reject malformed requests before they reach WEBrick
• Use network-level controls to restrict direct access to WEBrick instances, ensuring all traffic passes through properly configured proxies
• As a temporary measure, restrict WEBrick usage to development and testing environments only until patching is complete

# Example nginx configuration to reject ambiguous requests
# Add to nginx server block to mitigate HTTP request smuggling
if ($http_transfer_encoding ~* "chunked" ) {
    set $smuggle_check "${smuggle_check}TE";
}
if ($http_content_length) {
    set $smuggle_check "${smuggle_check}CL";
}
if ($smuggle_check = "TECL") {
    return 400;
}