CVE-2024-6866 Overview
CVE-2024-6866 affects corydolphin/flask-cors version 4.0.1, a popular Cross-Origin Resource Sharing (CORS) extension for Flask applications. The vulnerability stems from case-insensitive URL path matching caused by reuse of the try_match function originally designed for host matching. Because URL paths are case-sensitive per RFC 3986 but the regex matching treats them as case-insensitive, attackers can request restricted paths using altered capitalization. This mismatch allows unauthorized origins to bypass CORS restrictions and access endpoints intended to be limited. The flaw is tracked under [CWE-178: Improper Handling of Case Sensitivity].
Critical Impact
Unauthorized cross-origin requests can reach restricted Flask endpoints, leading to sensitive data exposure across applications relying on Flask-CORS path-based access controls.
Affected Products
- corydolphin/flask-cors version 4.0.1
- Python web applications using Flask-CORS for origin restriction
- Debian Long Term Support distributions packaging vulnerable flask-cors releases
Discovery Timeline
- 2025-03-20 - CVE-2024-6866 published to the National Vulnerability Database
- 2025-11-03 - Last updated in NVD database
Technical Details for CVE-2024-6866
Vulnerability Analysis
Flask-CORS allows developers to restrict cross-origin access on a per-path basis using regex resource patterns. In version 4.0.1, the library evaluates incoming request paths against these patterns through the try_match helper. That helper was implemented for host matching, where case insensitivity is correct behavior under RFC 3986. Applying the same logic to URL paths violates the standard, since path components must be compared as case-sensitive strings.
As a result, a CORS policy intended to protect /Admin will also match requests to /admin, /ADMIN, or any case variant. Attackers controlling an untrusted origin can craft requests targeting case-shifted paths that fall outside the configured allowlist but still match the regex used to grant CORS headers. The browser then accepts the response, exposing data from the protected endpoint.
Root Cause
The root cause is incorrect reuse of host-matching semantics for path comparison. The try_match function compiles the resource pattern with case-insensitive flags. Path-based access controls built on top of Flask-CORS therefore fail closed only for the exact casing developers anticipated, leaving variants exploitable.
Attack Vector
Exploitation is network-based and requires no authentication or user interaction. An attacker hosts a malicious origin that issues fetch or XMLHttpRequest calls to a target Flask application, varying the case of restricted path segments. When Flask-CORS returns permissive Access-Control-Allow-Origin headers due to the case-insensitive match, the browser releases the response body to the attacker's script, enabling data theft.
No verified public exploit code is available. See the Huntr Bounty Report for technical details from the original disclosure.
Detection Methods for CVE-2024-6866
Indicators of Compromise
- Access logs showing requests to case-shifted variants of restricted paths (e.g., /Admin, /aDmIn) followed by successful 200 responses with CORS headers.
- Origin request headers from domains not present in the application's documented allowlist.
- Repeated preflight OPTIONS requests probing capitalization variants of sensitive endpoints.
Detection Strategies
- Inventory Python dependencies and flag any installation of flask-cors==4.0.1 or earlier vulnerable releases.
- Inspect Flask-CORS resource configurations for regex patterns that rely on case-sensitive path enforcement.
- Replay restricted endpoint requests with altered casing in a staging environment and confirm whether CORS headers are returned.
Monitoring Recommendations
- Forward web server and WAF logs to a centralized analytics platform and alert on anomalous Origin headers paired with sensitive paths.
- Track outbound responses containing Access-Control-Allow-Origin for restricted routes that should never be exposed cross-origin.
- Baseline normal path casing for each endpoint and alert when deviations appear in production traffic.
How to Mitigate CVE-2024-6866
Immediate Actions Required
- Upgrade flask-cors to a fixed release beyond 4.0.1 as published by the maintainers and downstream distributions.
- Audit all Flask-CORS resource patterns and replace path regexes that depend on exact casing with explicit allowlists.
- For Debian systems, apply the update referenced in the Debian LTS Announcement.
Patch Information
The Flask-CORS maintainers addressed the issue in releases following 4.0.1 by correcting path comparison to be case-sensitive. Debian LTS published a fixed package through the May 2025 advisory. Review the Huntr Bounty Report for the upstream remediation reference.
Workarounds
- Normalize incoming request paths to a canonical case at a reverse proxy and reject variants that do not match documented routes.
- Implement server-side authorization checks on sensitive endpoints rather than relying solely on CORS for access control.
- Restrict allowed origins to a strict allowlist and avoid wildcard * configurations until patched.
# Configuration example: upgrade flask-cors to a patched release
pip install --upgrade 'flask-cors>4.0.1'
pip show flask-cors | grep -i version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
