CORS Checker & Access Control Validator

Perform deep architectural validation of Cross-Origin Resource Sharing (CORS) policies. Analyze Access-Control-Allow-Origin headers, preflight OPTIONS requests, and credentialed cross-origin access configurations against the Fetch standard and RFC 6454.

No signup

Instant results

100% free

Why Strict CORS Validation Matters

CORS is a fundamental browser security mechanism that relaxes the Same-Origin Policy (SOP). Misconfigurations, such as echoing the Origin header with Access-Control-Allow-Credentials set to true, or allowing wildcard origins inappropriately, can lead to severe data exfiltration vulnerabilities, cross-site request forgery (CSRF), and unauthorized API access.

Why It Matters

Common Mistakes

Dynamically echoing the Origin header blindly without whitelist validation.

INPUT_RECEIVED

QUERY_SENT

ANALYZING_DATA

CHECK_PASSED

ENGINE_ACTIVE

RESULT_FOUND

How It Works

Preflight Validation

Simulates an HTTP OPTIONS request to evaluate Access-Control-Allow-Methods and Access-Control-Allow-Headers returned by your server.

Origin Evaluation

Tests multiple origin payloads including null, malicious subdomains, and wildcard boundaries to detect blind Origin reflection.

Credential Verification

Analyzes the interaction between Access-Control-Allow-Credentials and your allowed origins to prevent authenticated data leaks.

Header Exposure Analysis

Checks the Access-Control-Expose-Headers directive to ensure sensitive custom headers are not inadvertently exposed to cross-origin JavaScript.

Preflight OPTIONS Simulation

Accurately models complex CORS preflight requests by sending mock Origin and Access-Control-Request-Method headers to evaluate your server's access control middleware.

Origin Reflection Detection

Identifies dangerous configurations where backend systems dynamically echo arbitrary Origin headers back in the Access-Control-Allow-Origin response.

Credential Leakage Prevention

Validates that your API does not improperly combine wildcard origins ('*') with credentialed access (cookies, TLS client certificates, or authorization headers).

Header Policy Validation

Analyzes Access-Control-Allow-Headers and Access-Control-Expose-Headers to prevent both request blocking and accidental data exposure.

Fetch Standard Compliance

Ensures your CORS implementation strictly adheres to the WHATWG Fetch Living Standard and RFC 6454 (The Web Origin Concept) guidelines.

Max-Age Optimization

Evaluates the Access-Control-Max-Age directive to optimize browser caching of preflight responses, minimizing latency for subsequent cross-origin requests.

Frequently Asked Questions

What is CORS and why is it necessary?

Cross-Origin Resource Sharing (CORS) is an HTTP-header based mechanism that allows a server to indicate any origins (domain, scheme, or port) other than its own from which a browser should permit loading resources. It acts as a controlled relaxation of the Same-Origin Policy (SOP).

How does a CORS preflight request work?

A preflight request is an HTTP OPTIONS request sent by the browser before the actual request. It includes the Origin, Access-Control-Request-Method, and Access-Control-Request-Headers. The server must respond with appropriate Access-Control-Allow-* headers for the browser to proceed with the actual request.

Why is it dangerous to set Access-Control-Allow-Origin to '*'?

Setting the allowed origin to a wildcard ('*') allows any website to access the resource. While suitable for public APIs, it becomes a critical vulnerability if the endpoint handles sensitive, user-specific data, especially if paired with authorization bypass techniques.

Can I use a wildcard origin with credentials?

No, the Fetch specification explicitly forbids using a wildcard ('*') in Access-Control-Allow-Origin when Access-Control-Allow-Credentials is set to 'true'. Doing so will cause the browser to reject the response and fail the cross-origin request.

What is Origin reflection and why is it a security risk?

Origin reflection occurs when a server dynamically reads the 'Origin' header from the request and mirrors it directly into the Access-Control-Allow-Origin response header without validation. This effectively grants full cross-origin access to any requesting domain, defeating the purpose of CORS.

How do I securely configure multiple allowed origins?

To support multiple origins, your server-side code must read the incoming Origin header, check it against a strictly defined whitelist of trusted domains (using exact matches, not loose regex), and if validated, return that specific origin in the Access-Control-Allow-Origin response header.

What happens if a preflight request returns a 401 Unauthorized status?

According to the HTTP standard, browsers expect a 2xx success status code for preflight OPTIONS requests. If your server enforces authentication on OPTIONS requests and returns a 401 or 403, the browser will fail the preflight, and the actual cross-origin request will never be sent.