CVE-2020-26137: Python Urllib3 CRLF Injection Vulnerability

CVE-2020-26137 Overview

CVE-2020-26137 is a CRLF injection vulnerability in Python's urllib3 library versions prior to 1.25.9. This vulnerability allows an attacker who controls the HTTP request method to inject CR (Carriage Return) and LF (Line Feed) control characters through the first argument of the putrequest() function. The flaw enables HTTP request smuggling and response splitting attacks, potentially leading to cache poisoning, session hijacking, or cross-site scripting.

Critical Impact

Attackers can inject arbitrary HTTP headers or manipulate request structure by embedding CRLF characters in the HTTP method parameter, potentially enabling HTTP response splitting attacks against downstream applications.

Affected Products

• Python urllib3 (versions prior to 1.25.9)
• Canonical Ubuntu Linux 16.04, 18.04, 20.04
• Debian Linux 9.0
• Oracle Communications Cloud Native Core Network Function Cloud Native Environment 22.2.0
• Oracle ZFS Storage Appliance Kit 8.8

Discovery Timeline

• September 30, 2020 - CVE-2020-26137 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-26137

Vulnerability Analysis

This CRLF injection vulnerability exists in urllib3's HTTP request handling mechanism. When processing HTTP requests, the library fails to properly validate and sanitize control characters in the HTTP method parameter passed to putrequest(). An attacker who can control or influence the HTTP method argument can inject CRLF sequences (\r\n) to break out of the intended HTTP request structure.

The vulnerability is similar to CVE-2020-26116, which affected Python's standard library http.client. In both cases, the lack of input validation on the method parameter allows malicious injection of HTTP headers or even entire HTTP requests, leading to request smuggling scenarios.

Root Cause

The root cause is classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component). The urllib3 library's HTTPConnection.request() method did not validate the method parameter for control characters before incorporating it into the HTTP request. This lack of input sanitization allows CRLF characters to be interpreted as HTTP protocol delimiters rather than literal content.

Attack Vector

The attack requires network access and the ability to influence the HTTP method parameter in urllib3 requests. An attacker can exploit this by:

1. Injecting \r\n (CRLF) sequences into the method parameter
2. The injected characters terminate the request line prematurely
3. Additional attacker-controlled content is interpreted as new HTTP headers or a separate request
4. This can lead to HTTP response splitting, cache poisoning, or session manipulation

The security patch introduced validation to raise a ValueError if control characters are detected in the method parameter:

 Changes
 =======
 
+master (dev)
+------------
+
+* Raise ``ValueError`` if control characters are given in
+  the ``method`` parameter of ``HTTPConnection.request()`` (Pull #1800)
+
+
 1.25.8 (2020-01-20)
 -------------------

Source: GitHub Commit for urllib3

The fix adds regex-based validation in src/urllib3/connection.py:

 from __future__ import absolute_import
+import re
 import datetime
 import logging
 import os

Source: GitHub Commit for urllib3

Detection Methods for CVE-2020-26137

Indicators of Compromise

• Unusual HTTP requests containing encoded CRLF sequences (%0d%0a or \r\n) in request method fields
• Web server logs showing malformed HTTP methods with embedded newlines
• HTTP responses containing injected headers not originating from the application
• Cache entries with unexpected or manipulated content

Detection Strategies

• Monitor application logs for HTTP requests with control characters in method fields
• Implement Web Application Firewall (WAF) rules to detect CRLF injection patterns in HTTP requests
• Scan Python environments for urllib3 versions below 1.25.9 using dependency scanning tools
• Review code for user-controlled input being passed to urllib3 HTTP method parameters

Monitoring Recommendations

• Deploy network-level monitoring to detect HTTP request smuggling patterns
• Enable verbose logging for HTTP client libraries in production environments
• Use Software Composition Analysis (SCA) tools to continuously monitor for vulnerable urllib3 versions
• Set up alerts for applications making HTTP requests with unusual method patterns

How to Mitigate CVE-2020-26137

Immediate Actions Required

• Upgrade urllib3 to version 1.25.9 or later immediately
• Audit applications for any code paths where user input could influence HTTP method parameters
• Apply vendor patches for affected Oracle and Linux distributions
• Review and update all Python dependencies that may bundle or depend on urllib3

Patch Information

The vulnerability was addressed in urllib3 version 1.25.9. The fix validates the HTTP method parameter and raises a ValueError if control characters are detected. Patches are available through the following resources:

• GitHub Commit for urllib3
• GitHub Pull Request #1800
• Ubuntu Security Notice USN-4570-1
• Debian LTS Security Announcement
• Oracle July 2022 Security Alert

Workarounds

• Implement strict input validation on any user-controllable data that may reach HTTP client methods
• Use application-level filtering to reject requests containing CRLF sequences before they reach urllib3
• If immediate patching is not possible, wrap urllib3 calls with validation logic that checks for control characters in method parameters
• Consider using network segmentation to limit the impact of potential HTTP request smuggling attacks

# Upgrade urllib3 to patched version
pip install --upgrade 'urllib3>=1.25.9'

# Verify installed version
pip show urllib3 | grep Version

# Check for vulnerable versions in requirements
pip list --outdated | grep urllib3