CVE-2020-25032: Flask-CORS Path Traversal Vulnerability

CVE-2020-25032 Overview

CVE-2020-25032 is a directory traversal vulnerability discovered in Flask-CORS (CORS Middleware for Flask) before version 3.0.9. The vulnerability allows attackers to access private resources through ../ directory traversal sequences because the resource matching mechanism does not ensure that pathnames are in a canonical format. This flaw enables unauthorized access to files and resources outside the intended directory scope.

Critical Impact

Attackers can exploit this directory traversal vulnerability to bypass CORS resource restrictions and access sensitive files or private resources on affected Flask applications.

Affected Products

• Flask-CORS versions prior to 3.0.9
• Debian Linux 10.0
• openSUSE Backports SLE 15.0 SP1 and SP2
• openSUSE Leap 15.1 and 15.2

Discovery Timeline

• August 31, 2020 - CVE-2020-25032 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25032

Vulnerability Analysis

This directory traversal vulnerability (CWE-22) exists in Flask-CORS due to improper path canonicalization during resource matching operations. When the middleware evaluates whether a request should be allowed based on CORS configuration, it fails to normalize pathnames before comparison. This oversight allows attackers to craft malicious requests containing ../ sequences that bypass the intended resource restrictions.

The vulnerability is exploitable remotely without authentication, making it accessible to any attacker who can send HTTP requests to an affected Flask application. The primary impact is confidentiality compromise, as attackers can read files and resources they should not have access to.

Root Cause

The root cause is the absence of path canonicalization in the resource matching logic within Flask-CORS. When comparing request paths against configured CORS resources, the middleware accepts non-canonical paths containing directory traversal sequences. This allows attackers to construct paths that appear to reference allowed resources but actually resolve to restricted locations after traversal.

The fix implemented in version 3.0.9 ensures that pathnames are properly normalized to their canonical format before resource matching occurs, preventing traversal sequences from bypassing access controls.

Attack Vector

The attack is network-based and requires no user interaction or special privileges. An attacker crafts HTTP requests to a Flask application using Flask-CORS middleware, including ../ sequences in the request path. Since the resource matching does not canonicalize paths, these traversal sequences allow the attacker to reference resources outside the allowed CORS scope.

For example, if CORS is configured to allow access only to /api/public/, an attacker could potentially access /api/private/ by crafting a request to /api/public/../private/. The middleware would incorrectly match the request against the allowed resource pattern before traversal resolution occurs.

Detection Methods for CVE-2020-25032

Indicators of Compromise

• HTTP request logs containing ../ or URL-encoded traversal sequences (%2e%2e%2f) in paths
• Unusual access patterns to resources that should be protected by CORS restrictions
• Web server logs showing requests attempting to access files outside normal application directories
• Error logs indicating file access attempts to unexpected paths

Detection Strategies

• Monitor web application logs for directory traversal patterns including ../, ..%2f, %2e%2e/, and similar encoded variations
• Implement web application firewall (WAF) rules to detect and block path traversal attempts
• Review Flask-CORS version across all deployed applications and flag versions below 3.0.9
• Conduct periodic security scans to identify applications running vulnerable Flask-CORS versions

Monitoring Recommendations

• Enable verbose logging for Flask applications to capture full request paths
• Configure alerting for suspicious path patterns in HTTP requests
• Monitor for unauthorized access to sensitive files or API endpoints
• Implement intrusion detection rules for directory traversal attack signatures

How to Mitigate CVE-2020-25032

Immediate Actions Required

• Upgrade Flask-CORS to version 3.0.9 or later immediately
• Review application logs for any evidence of exploitation attempts
• Audit CORS configurations to ensure they follow the principle of least privilege
• Implement input validation to reject requests containing directory traversal sequences

Patch Information

The vulnerability is fixed in Flask-CORS version 3.0.9. The fix ensures that pathnames are canonicalized before resource matching operations. Update your Flask-CORS dependency using your package manager:

Security advisories are available from multiple sources:

• Flask-CORS 3.0.9 Release Notes on GitHub
• Debian Security Advisory DSA-4775
• openSUSE Security Announcements

Workarounds

• Implement additional path validation middleware that normalizes and validates request paths before they reach Flask-CORS
• Configure reverse proxy or web server rules to block requests containing directory traversal sequences
• Use web application firewalls (WAF) to filter malicious requests at the network perimeter
• Restrict file system permissions to limit the impact of potential traversal attacks

# Upgrade Flask-CORS to patched version
pip install --upgrade flask-cors>=3.0.9

# Verify installed version
pip show flask-cors | grep Version