CVE-2022-40899: Python-future DOS Vulnerability

CVE-2022-40899 Overview

CVE-2022-40899 is a Denial of Service vulnerability discovered in Python Charmers Future version 0.18.2 and earlier. The vulnerability allows remote attackers to cause a denial of service via a crafted Set-Cookie header from a malicious web server. This is classified as a Regular Expression Denial of Service (ReDoS) vulnerability that can lead to resource exhaustion when processing specially crafted cookie data.

Critical Impact

Remote attackers can exhaust server resources by sending malicious Set-Cookie headers, causing applications using the vulnerable python-future library to become unresponsive and potentially crash.

Affected Products

• Python Charmers python-future version 0.18.2 and earlier
• Applications using the future.backports.http.cookiejar module
• Python 2/3 compatibility layer implementations relying on python-future

Discovery Timeline

• 2022-12-23 - CVE-2022-40899 published to NVD
• 2025-04-15 - Last updated in NVD database

Technical Details for CVE-2022-40899

Vulnerability Analysis

This vulnerability is a Regular Expression Denial of Service (ReDoS) affecting the cookie handling functionality in the python-future library. The issue resides in the cookiejar.py module, specifically in the regex pattern used to parse Set-Cookie headers. When a malicious web server sends a specially crafted Set-Cookie header, the vulnerable regular expression can enter a state of catastrophic backtracking, consuming excessive CPU resources and causing the application to hang or become unresponsive.

The python-future library is designed to provide a compatibility layer for writing Python code that works on both Python 2 and Python 3. The vulnerable code is part of the backported HTTP cookie handling functionality, making any application that processes cookies from untrusted sources potentially vulnerable.

Root Cause

The root cause of this vulnerability is an inefficient regular expression pattern in the cookie parsing logic located in future/backports/http/cookiejar.py. The regex pattern contains constructs that allow for exponential time complexity when processing certain input strings, a class of vulnerability known as CWE-400 (Uncontrolled Resource Consumption). When the regex engine attempts to match a maliciously crafted cookie header, it performs excessive backtracking operations, leading to CPU exhaustion.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. An attacker can exploit this vulnerability by setting up a malicious web server that responds with specially crafted Set-Cookie headers. When a vulnerable application makes an HTTP request to the malicious server and processes the response cookies, the ReDoS condition is triggered.

The attack scenario typically involves:

1. An attacker controls or compromises a web server
2. The attacker configures the server to respond with malicious Set-Cookie headers
3. A victim application using python-future makes an HTTP request to this server
4. The cookie parsing regex enters catastrophic backtracking
5. The application's CPU usage spikes and the service becomes unresponsive

The vulnerable regex pattern is located in the cookiejar.py module. The fix was implemented in GitHub Pull Request #610, which optimizes the regular expression to prevent catastrophic backtracking.

Detection Methods for CVE-2022-40899

Indicators of Compromise

• Abnormally high CPU utilization on systems running Python applications
• Application threads hanging during HTTP response processing
• Network connections timing out when processing responses from specific servers
• Process monitoring showing extended execution time in Python regex operations

Detection Strategies

• Monitor application CPU usage for sudden spikes correlated with HTTP client operations
• Implement request timeouts for external HTTP calls to detect hanging operations
• Use application performance monitoring (APM) tools to identify slow cookie parsing operations
• Scan application dependencies for python-future versions 0.18.2 and earlier using software composition analysis tools

Monitoring Recommendations

• Deploy SentinelOne Singularity platform to monitor for anomalous process behavior and resource exhaustion patterns
• Implement application-level logging for HTTP response processing times
• Set up alerts for CPU exhaustion patterns in Python application processes
• Use dependency scanning in CI/CD pipelines to identify vulnerable library versions

How to Mitigate CVE-2022-40899

Immediate Actions Required

• Upgrade python-future to a patched version that includes the fix from Pull Request #610
• Audit all Python applications for usage of python-future library
• Implement HTTP request timeouts to prevent indefinite hanging
• Consider migrating to Python 3 native libraries to eliminate dependency on python-future

Patch Information

The vulnerability was addressed through a fix to the regular expression pattern in the cookiejar module. The patch is available via GitHub Pull Request #610. This fix aligns with the corresponding fix in CPython itself, documented in cpython Pull Request #17157. Users should update to the latest version of python-future available on PyPI.

For additional technical analysis of this and related ReDoS vulnerabilities in Python packages, refer to the Pyup security research.

Workarounds

• Implement connection timeouts on all outbound HTTP requests to limit exposure to malicious servers
• Use input validation to sanitize or reject abnormally long Set-Cookie headers before processing
• Consider using alternative cookie handling libraries that are not affected by this vulnerability
• Deploy network-level controls to limit connections to untrusted external servers

# Update python-future to the latest patched version
pip install --upgrade future

# Verify the installed version
pip show future | grep Version

# Scan for vulnerable dependencies in your project
pip-audit --desc