CVE-2020-25613: Ruby WEBrick HTTP Request Smuggling Flaw

CVE-2020-25613 Overview

An HTTP Request Smuggling vulnerability was discovered in Ruby's WEBrick, a simple HTTP server bundled with the Ruby programming language. The vulnerability exists because WEBrick did not rigorously check the transfer-encoding header value. An attacker may potentially exploit this issue to bypass a reverse proxy (which also has a poor header check), leading to an HTTP Request Smuggling attack that could allow unauthorized access, cache poisoning, or request hijacking.

Critical Impact

Attackers can exploit lax header validation to smuggle malicious HTTP requests through reverse proxies, potentially bypassing security controls and manipulating backend server behavior.

Affected Products

• Ruby through 2.5.8
• Ruby 2.6.x through 2.6.6
• Ruby 2.7.x through 2.7.1
• WEBrick (bundled with affected Ruby versions)
• Fedora 32 and 33

Discovery Timeline

• 2020-10-06 - CVE-2020-25613 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25613

Vulnerability Analysis

This vulnerability falls under CWE-444 (Inconsistent Interpretation of HTTP Requests), commonly known as HTTP Request Smuggling. The flaw resides in WEBrick's HTTP request parsing logic, specifically in how it interprets HTTP headers like Connection and Transfer-Encoding.

WEBrick's implementation used overly permissive regular expressions when parsing critical HTTP headers. The regex patterns /close/io and /keep-alive/io would match these strings anywhere within the header value, rather than requiring exact matches. This loose validation allows an attacker to craft ambiguous HTTP requests that are interpreted differently by WEBrick and intermediate proxies.

When a reverse proxy and backend server disagree on where one request ends and another begins, attackers can "smuggle" requests that bypass front-end security controls. This can lead to unauthorized access to restricted resources, web cache poisoning, session hijacking, or bypassing of WAF protections.

Root Cause

The root cause is insufficient input validation of HTTP header values in WEBrick's httprequest.rb file. The regular expressions used to parse the Connection header were not anchored, allowing partial matches. For example, a malicious header value like close, smuggled-data would incorrectly match the /close/io pattern, causing inconsistent request boundary interpretation between the proxy and backend server.

Attack Vector

The attack requires network access to a Ruby application using WEBrick behind a reverse proxy. The attacker sends specially crafted HTTP requests with ambiguous header values that exploit the difference in how the proxy and WEBrick parse requests. This network-based attack requires no authentication or user interaction.

# Security patch from lib/webrick/httprequest.rb
# Source: https://github.com/ruby/webrick/commit/8946bb38b4d87549f0d99ed73c62c41933f97cc7

         raise HTTPStatus::BadRequest, "bad URI `#{@unparsed_uri}'."
       end
 
-      if /close/io =~ self["connection"]
+      if /\Aclose\z/io =~ self["connection"]
         @keep_alive = false
-      elsif /keep-alive/io =~ self["connection"]
+      elsif /\Akeep-alive\z/io =~ self["connection"]
         @keep_alive = true
       elsif @http_version < "1.1"
         @keep_alive = false

The fix adds anchors (\A for start of string, \z for end of string) to the regular expressions, ensuring exact matches of header values rather than partial matches.

Detection Methods for CVE-2020-25613

Indicators of Compromise

• Unusual HTTP traffic patterns with malformed or suspicious Transfer-Encoding or Connection headers
• Log entries showing discrepancies between proxy access logs and backend server logs
• Requests with multiple or conflicting Transfer-Encoding values (e.g., chunked, identity)
• Evidence of cache poisoning or unexpected content being served to users

Detection Strategies

• Monitor HTTP request logs for anomalous header patterns, particularly in Transfer-Encoding and Connection headers
• Implement intrusion detection rules to flag requests with potentially malicious header combinations
• Compare front-end proxy logs with backend WEBrick logs to identify request boundary discrepancies
• Deploy web application firewalls (WAF) with HTTP Request Smuggling detection capabilities

Monitoring Recommendations

• Enable detailed HTTP request logging on both reverse proxies and WEBrick backends
• Set up alerts for requests containing unusual header value combinations
• Monitor for signs of cache poisoning such as unexpected content changes or user complaints
• Regularly audit Ruby and WEBrick versions across all deployment environments

How to Mitigate CVE-2020-25613

Immediate Actions Required

• Upgrade Ruby to version 2.5.9, 2.6.7, 2.7.2, or later which contain the security fix
• Update the WEBrick gem to the patched version if using it as a standalone dependency
• Review reverse proxy configurations to ensure strict HTTP header validation
• Consider replacing WEBrick with a more hardened HTTP server for production use

Patch Information

The vulnerability has been addressed in the official Ruby releases. The fix involves modifying the regular expressions in lib/webrick/httprequest.rb to use anchored patterns that require exact header value matches. The security patch is available in the GitHub commit. For detailed information, refer to the Ruby Security Advisory.

Workarounds

• Configure reverse proxies to normalize and strictly validate Transfer-Encoding and Connection headers before forwarding requests
• Implement request filtering at the proxy level to reject ambiguous HTTP requests
• Use a WAF or similar security appliance to detect and block HTTP Request Smuggling attempts
• Replace WEBrick with alternative servers like Puma or Unicorn in production environments

# Configuration example - Update Ruby using rbenv
rbenv install 2.7.2
rbenv global 2.7.2
ruby -v  # Verify version

# Or update via package manager (Fedora)
sudo dnf update ruby

# Update WEBrick gem if used standalone
gem update webrick