CVE-2020-26116 Overview
CVE-2020-26116 is a CRLF injection vulnerability in Python's http.client module that allows attackers to inject arbitrary HTTP headers when they control the HTTP request method. The vulnerability exists in Python 3.x versions before 3.5.10, 3.6.x before 3.6.12, 3.7.x before 3.7.9, and 3.8.x before 3.8.5. By inserting carriage return (CR) and line feed (LF) control characters in the first argument of HTTPConnection.request(), attackers can manipulate HTTP requests to perform HTTP header injection attacks.
Critical Impact
Attackers can inject malicious HTTP headers, potentially enabling request smuggling, cache poisoning, or session hijacking attacks against applications using vulnerable Python versions.
Affected Products
- Python 3.x before 3.5.10, 3.6.x before 3.6.12, 3.7.x before 3.7.9, 3.8.x before 3.8.5
- Fedora 31, 32, 33
- Canonical Ubuntu Linux 12.04, 14.04 ESM, 16.04 ESM, 18.04 LTS
- NetApp SolidFire, HCI Compute Node, HCI Storage Node
- Debian Linux 9.0
- Oracle ZFS Storage Appliance Kit 8.8
- openSUSE Leap 15.1
Discovery Timeline
- 2020-09-27 - CVE-2020-26116 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-26116
Vulnerability Analysis
The vulnerability resides in Python's http.client module, which is part of the standard library used extensively for making HTTP requests. The core issue is that the module fails to properly validate and sanitize the HTTP method parameter passed to the HTTPConnection.request() function. HTTP methods should consist only of valid token characters, but the vulnerable implementation allows CR (\r) and LF (\n) control characters to pass through without rejection.
When these CRLF sequences are present in the method parameter, they are interpreted by HTTP servers as header delimiters, allowing an attacker to inject additional HTTP headers or even craft entirely separate HTTP requests. This weakness is classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component), commonly known as an injection vulnerability.
Root Cause
The root cause is insufficient input validation in the HTTPConnection.request() method. The implementation does not verify that the HTTP method argument contains only permitted characters as defined by RFC 7230. Specifically, HTTP token characters should not include control characters like CR and LF. The absence of this validation allows attackers who can influence the method parameter to break out of the expected HTTP structure and inject arbitrary content into the request.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker must be able to control or influence the HTTP request method parameter in a Python application. This commonly occurs in scenarios where:
- Applications dynamically construct HTTP methods based on user input
- Web applications proxy or forward requests with user-controlled parameters
- APIs accept method specifications from external sources
When exploited, the attacker injects CRLF characters followed by malicious headers. For example, passing a method like GET / HTTP/1.1\r\nX-Injected: malicious\r\n\r\nGET would cause the Python HTTP client to send what appears to be two separate requests, potentially poisoning caches, hijacking sessions, or bypassing security controls.
The vulnerability is exploited by crafting a malicious method string containing CRLF sequences followed by the desired injected headers. When the application passes this to HTTPConnection.request(), the injected content becomes part of the raw HTTP request sent to the server. For detailed technical analysis, refer to the Python Security Vulnerability Documentation.
Detection Methods for CVE-2020-26116
Indicators of Compromise
- Unusual HTTP request patterns in web server logs containing unexpected method names with special characters
- HTTP requests with method fields containing %0d%0a (URL-encoded CRLF) or raw CRLF sequences
- Application logs showing HTTP client errors related to malformed requests
- Evidence of HTTP response splitting or cache poisoning attacks targeting Python-based services
Detection Strategies
- Implement web application firewall (WAF) rules to detect CRLF sequences in HTTP method fields
- Monitor application logs for HTTP 400 Bad Request responses that may indicate injection attempts
- Deploy network intrusion detection systems (IDS) with signatures for HTTP header injection patterns
- Audit Python applications for user-controllable input flowing into HTTPConnection.request() method parameters
Monitoring Recommendations
- Enable verbose logging in Python HTTP client applications to capture full request details
- Configure centralized log aggregation to correlate suspicious HTTP activity across services
- Establish baseline HTTP traffic patterns to identify anomalous request methods
How to Mitigate CVE-2020-26116
Immediate Actions Required
- Upgrade Python to patched versions: 3.5.10+, 3.6.12+, 3.7.9+, or 3.8.5+
- Audit application code for any instances where user input influences HTTP method parameters
- Implement input validation to reject HTTP methods containing control characters
- Apply vendor-specific security patches from distribution maintainers
Patch Information
Security patches have been released by Python and various Linux distributions. The fix ensures that HTTP methods are properly validated to reject control characters before being included in HTTP requests. Refer to the following resources for patching:
- Python Bug Report #39603 - Original bug report and fix details
- Ubuntu Security Notice USN-4581-1 - Ubuntu patched packages
- Gentoo GLSA 202101-18 - Gentoo security advisory
- NetApp Security Advisory - NetApp affected products
Workarounds
- Sanitize all user input before passing to HTTPConnection.request() by stripping or rejecting CR and LF characters
- Use application-level input validation to ensure HTTP methods contain only alphanumeric characters
- Consider wrapping HTTP client calls with a validation layer that checks method parameters
# Example: Check Python version and upgrade if vulnerable
python3 --version
# If version is below 3.5.10, 3.6.12, 3.7.9, or 3.8.5, upgrade:
# For Debian/Ubuntu:
sudo apt update && sudo apt upgrade python3
# For RHEL/CentOS/Fedora:
sudo dnf update python3
# Verify updated version:
python3 --version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
