CVE-2024-6221: Flask-CORS Information Disclosure Flaw

CVE-2024-6221 Overview

A vulnerability in corydolphin/flask-cors version 4.0.1 allows the Access-Control-Allow-Private-Network CORS header to be set to true by default. This behavior can expose private network resources to unauthorized external access, leading to significant security risks such as data breaches, unauthorized access to sensitive information, and potential network intrusions.

Critical Impact

Default CORS misconfiguration enables external websites to access private network resources, potentially exposing internal APIs and sensitive data to unauthorized parties.

Affected Products

• corydolphin flask-cors version 4.0.1

Discovery Timeline

• 2024-08-18 - CVE CVE-2024-6221 published to NVD
• 2025-04-07 - Last updated in NVD database

Technical Details for CVE-2024-6221

Vulnerability Analysis

This vulnerability stems from an insecure default configuration in the Flask-CORS extension. The Access-Control-Allow-Private-Network header, introduced as part of the Private Network Access (PNA) specification, is designed to provide an additional layer of protection for resources on private networks. When this header is set to true, it explicitly permits cross-origin requests from public websites to access resources on private networks (such as localhost, intranet servers, or devices on local networks).

In Flask-CORS version 4.0.1, this header defaults to true, which violates the principle of secure defaults. This misconfiguration creates a pathway for malicious websites to initiate requests to internal services that should not be accessible from the public internet. The vulnerability is categorized under CWE-284 (Improper Access Control).

Root Cause

The root cause is an insecure default configuration where the Access-Control-Allow-Private-Network CORS header is automatically set to true without requiring explicit opt-in by the developer. This default setting contradicts security best practices, which mandate that access to private network resources should be explicitly granted rather than implicitly allowed.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can craft a malicious website that makes cross-origin requests to internal services. When a victim visits the attacker's website while connected to a private network, the browser will include the Access-Control-Request-Private-Network header in preflight requests. Due to the vulnerable Flask-CORS configuration automatically responding with Access-Control-Allow-Private-Network: true, the browser permits the cross-origin request to proceed, granting the attacker access to internal resources.

This attack pattern enables reconnaissance of internal network services, exfiltration of sensitive data from internal APIs, and potential exploitation of internal services that lack robust authentication mechanisms because they were designed to be accessible only from trusted internal networks.

Detection Methods for CVE-2024-6221

Indicators of Compromise

• Unexpected cross-origin requests to internal Flask applications from external domains
• HTTP response headers containing Access-Control-Allow-Private-Network: true without explicit configuration
• Unusual traffic patterns from external IPs accessing internal services
• Web server logs showing preflight OPTIONS requests with Access-Control-Request-Private-Network headers from untrusted origins

Detection Strategies

• Audit Flask-CORS configurations in all Python applications to verify Access-Control-Allow-Private-Network settings
• Implement network monitoring to detect cross-origin requests targeting internal services
• Review web application firewall logs for suspicious CORS-related request patterns
• Conduct dependency scanning to identify applications running Flask-CORS version 4.0.1

Monitoring Recommendations

• Configure alerts for unusual CORS preflight request volumes to internal services
• Monitor for responses containing Access-Control-Allow-Private-Network: true headers
• Implement logging of all cross-origin requests to sensitive internal endpoints
• Deploy network segmentation monitoring to detect unauthorized cross-network access attempts

How to Mitigate CVE-2024-6221

Immediate Actions Required

• Upgrade Flask-CORS to a patched version that does not enable Access-Control-Allow-Private-Network by default
• Explicitly set Access-Control-Allow-Private-Network to false in your Flask-CORS configuration
• Review and restrict CORS origins configuration to only trusted domains
• Audit all Flask applications for exposure to this misconfiguration

Patch Information

A fix has been committed to the Flask-CORS repository. The relevant commit can be found in the GitHub Commit Log. Organizations should update their Flask-CORS dependency to incorporate this fix. Additional details about the vulnerability discovery are available in the Huntr Bounty Announcement.

Workarounds

• Explicitly configure Flask-CORS to disable the Access-Control-Allow-Private-Network header in your application settings
• Implement network-level controls to restrict external access to internal services
• Use a reverse proxy or web application firewall to filter or modify CORS headers before they reach clients
• Consider network segmentation to isolate sensitive internal services from externally-facing applications

# Configuration example for Flask application
# Explicitly disable Access-Control-Allow-Private-Network in Flask-CORS configuration
# Add to your Flask application initialization:
# CORS(app, supports_credentials=False, private_network=False)
# 
# Or via environment variable / config file:
export FLASK_CORS_PRIVATE_NETWORK=false