Flask 3.1.2 Vulnerability: CVE-2025-66221 Security Alert

by Alex Johnson 57 views

Is your Flask application at risk? A vulnerability has been identified in the flask-3.1.2-py3-none-any.whl package, with a medium severity rating and a CVSS score of 5.3. This article dives deep into the details of this vulnerability, its potential impact, and how to mitigate the risk. Stay informed and ensure your applications are secure!

Understanding the Vulnerability

The vulnerability, tracked as CVE-2025-66221, affects the werkzeug library, specifically version 3.1.3, which is a dependency of flask-3.1.2. Werkzeug is a comprehensive WSGI (Web Server Gateway Interface) web application library for Python. This particular vulnerability arises from how Werkzeug's safe_join function handles path segments on Windows systems.

The Technical Details

Werkzeug's safe_join function is designed to safely join a base path with a user-provided path segment, preventing directory traversal vulnerabilities. However, prior to version 3.1.4, this function incorrectly handled path segments containing Windows device names. Windows reserves certain names like CON, AUX, PRN, and others as device names, which are implicitly present in every directory. When an application uses send_from_directory, which relies on safe_join, to serve files, a request ending with one of these device names on a Windows system could lead to a denial-of-service (DoS) condition. The file would open successfully, but any attempt to read from it would hang indefinitely.

Impact and Severity

The medium severity rating (CVSS 5.3) reflects the potential impact of this vulnerability. While it doesn't directly expose confidential data or allow for unauthorized modification, it can lead to a denial-of-service, disrupting the availability of the application. The attack vector is over the network, and the attack complexity is low, meaning it's relatively easy to exploit. No privileges or user interaction are required, making it a concern for applications running on Windows servers.

Identifying the Vulnerable Component

The vulnerable library is werkzeug-3.1.3-py3-none-any.whl, a transitive dependency of flask-3.1.2-py3-none-any.whl. The path to the dependency file is typically found in your project's requirements.txt file. The vulnerable library itself resides within the site-packages directory of your Python environment, specifically in a path similar to /tmp/ws-ua_.../python_PYMFXB/.../env/lib/python3.11/site-packages/werkzeug-3.1.3.dist-info.

Dependency Hierarchy

To illustrate the dependency chain:

  • Your Root Library: flask-3.1.2-py3-none-any.whl
  • Vulnerable Transitive Dependency: werkzeug-3.1.3-py3-none-any.whl

If your project uses flask-3.1.2 and is running on a Windows server, it's crucial to address this vulnerability.

Remediation: Upgrading Werkzeug

The recommended solution is to upgrade Werkzeug to version 3.1.4 or later. This version includes a patch that resolves the vulnerability by correctly handling Windows device names in the safe_join function.

Steps to Upgrade

  1. Check your requirements.txt: If you explicitly list werkzeug in your requirements.txt file, update the version specification to werkzeug>=3.1.4.

  2. If Werkzeug is not explicitly listed: Werkzeug is a dependency of Flask, so upgrading Flask to a version that depends on Werkzeug 3.1.4 or later will resolve the issue. However, ensure compatibility with your existing codebase before upgrading Flask.

  3. Use pip to upgrade: Run the following command in your terminal, within your project's virtual environment:

    pip install -U werkzeug>=3.1.4

    Or, if you choose to upgrade Flask:

    pip install -U flask

  4. Verify the upgrade: After the upgrade, verify that Werkzeug version 3.1.4 or later is installed by running:

    pip show werkzeug

    This command will display information about the installed Werkzeug package, including its version.

Understanding the Fix

The fix for CVE-2025-66221 was implemented in this commit on the Werkzeug GitHub repository. The changes ensure that safe_join correctly validates path segments, preventing the vulnerability on Windows systems.

Additional Security Best Practices

While upgrading Werkzeug resolves this specific vulnerability, it's essential to adopt broader security practices to protect your Flask applications. Here are some key recommendations:

  1. Regularly Update Dependencies: Keep your project's dependencies up-to-date to benefit from security patches and bug fixes. Use tools like pip-tools or Poetry to manage your dependencies effectively.
  2. Implement Input Validation: Validate all user inputs to prevent various types of attacks, including injection vulnerabilities and path traversal attacks. Use Werkzeug's secure utilities for handling file paths and URLs.
  3. Use a Web Application Firewall (WAF): A WAF can help protect your application from common web attacks, such as SQL injection, cross-site scripting (XSS), and more.
  4. Security Audits: Conduct regular security audits and penetration testing to identify potential vulnerabilities in your application.
  5. Monitor for Vulnerabilities: Use vulnerability scanning tools and services to continuously monitor your dependencies for known vulnerabilities. Services like Mend.io and others can provide timely alerts about security issues in your project.

Conclusion

The CVE-2025-66221 vulnerability in werkzeug, a dependency of flask-3.1.2, poses a denial-of-service risk for Flask applications running on Windows. Upgrading to Werkzeug version 3.1.4 or later is the recommended solution. By understanding the details of this vulnerability, its potential impact, and the steps to mitigate it, you can ensure the security and availability of your Flask applications. Remember to also adopt broader security best practices to safeguard your applications against future threats.

For more information on web application security and best practices, visit the OWASP Foundation website.