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Access-Control-Allow-Origin Checker

Enter CORS response headers to check if your server's cross-origin policy is too permissive. Our validator analyzes Access-Control-Allow-Origin, detects dangerous configurations like wildcard with credentials, and provides a security grade (A+ to F) with actionable recommendations - all processed locally in your browser with complete privacy.

Access-Control-Allow-Origin Checker

Paste CORS headers to check if the server's cross-origin policy is too permissive. Detects wildcard origins with credentials, null origins, and other common misconfigurations.

Examples:

Paste CORS headers above or load an example to check their security posture

All validation happens locally in your browser - no data leaves your device

Why Use Our Access-Control-Allow-Origin Checker?

CORS Security Validation

Paste your CORS headers and instantly check if they follow security best practices. Our validator checks Access-Control-Allow-Origin, Access-Control-Allow-Credentials, Access-Control-Allow-Methods, Access-Control-Allow-Headers, Access-Control-Expose-Headers, and Access-Control-Max-Age against known security patterns.

Critical Misconfiguration Detection

Detects dangerous CORS configurations that can lead to security vulnerabilities: wildcard origins (*) combined with credentials, null origins, invalid origin URLs, trailing slashes on origins, and overly permissive header or method lists. Each issue includes a severity rating and clear explanation.

Instant Security Grade (A+ to F)

Get an immediate security grade from A+ (excellent) to F (critical issues) with a numeric score out of 100. The grade reflects the overall security posture of your CORS configuration and helps you prioritise which issues to address first.

100% Private & Client-Side

All CORS header validation is performed entirely in your browser using JavaScript. Your headers never leave your device - no uploads, no server processing, no logging. Check sensitive internal CORS configurations with complete privacy.

Common Use Cases for Access-Control-Allow-Origin Checker

API Security Auditing

Audit your API CORS configuration to ensure cross-origin requests are properly restricted. Check that sensitive endpoints do not use wildcard origins, that credentials are only allowed when explicitly needed, and that the allowed methods and headers are minimal.

CORS Misconfiguration Discovery

During security assessments, paste CORS headers from any server to quickly identify overly permissive configurations. Detect wildcard origins with credentials, null origins that enable sandboxed exploits, or invalid origin formats that cause browser errors.

Pre-Production CORS Verification

Before deploying a new API or web application, validate your CORS configuration against security best practices. Ensure that production CORS policies are properly scoped and that development-stage permissive configurations are not accidentally deployed.

Browser CORS Error Debugging

When debugging CORS errors in the browser, use this tool to check if the CORS headers returned by the server are correctly formatted. Validate that the origin matches, credentials are configured properly, and the Access-Control-Allow-Origin header is present.

Third-Party API CORS Check

Check the CORS configuration of third-party APIs you plan to integrate with. Verify that they allow your origin, that the response headers include the methods and headers you need, and that their CORS policy follows security best practices.

CDN & Static Asset Configuration

Validate CORS headers for CDN-hosted resources like fonts, scripts, and stylesheets. Ensure that cross-origin asset serving is properly configured so that browsers can load resources securely while maintaining performance.

Understanding CORS and Access-Control-Allow-Origin

What is the Access-Control-Allow-Origin Header?

The Access-Control-Allow-Origin (ACAO) header is a CORS (Cross-Origin Resource Sharing) response header that tells browsers whether a web application running at one origin can access resources from a different origin. When a browser makes a cross-origin request, it checks the ACAO header in the server response to determine if the requesting origin is allowed to read the response. If the header is missing or does not match the requesting origin, the browser blocks the response and throws a CORS error.

The ACAO header accepts either a specific origin (e.g., https://example.com), a wildcard (*) that allows any origin, ornull for special cases. The choice between specific origins and wildcards has significant security implications and must be carefully considered based on whether the API serves sensitive data.

How Our Access-Control-Allow-Origin Checker Works

Our tool parses CORS headers and evaluates them against security best practices.

  1. Header parsing - The tool reads the input and identifies up to 6 CORS response headers: Access-Control-Allow-Origin, Access-Control-Allow-Credentials, Access-Control-Allow-Methods, Access-Control-Allow-Headers, Access-Control-Expose-Headers, and Access-Control-Max-Age. It supports raw values (just the origin) and full header: value format.
  2. Security analysis - Each header is checked for security issues: wildcard origins with credentials (browser-rejected), null origins (sandbox exploit risk), invalid origin URLs, trailing slashes, duplicate methods, and inappropriate wildcard usage. Each finding is classified by severity (error, warning, info).
  3. Grade calculation - Based on the findings, a score from 0-100 is calculated and converted to a letter grade from A+ (excellent) to F (critical issues). The grade reflects the overall security posture of your CORS configuration.

Common CORS Security Risks

Understanding these common CORS misconfigurations helps you build more secure web applications:

  • Wildcard with credentials - Using Access-Control-Allow-Origin: * together with Access-Control-Allow-Credentials: true is explicitly forbidden by the CORS specification. Browsers will reject requests with this combination entirely. Use a specific origin when credentials are required.
  • Overly permissive origins - Using a wildcard (*) for the allowed origin exposes your API to any website. This is appropriate for public, non-sensitive data endpoints but should be avoided when authentication or user-specific data is involved.
  • Null origin allowance - Setting Access-Control-Allow-Origin: null allows sandboxed iframes and locally opened HTML files to make cross-origin requests. This can be exploited for local file-based attacks.
  • Reflected origins - If the server dynamically echoes the Origin request header without validation, it can allow any malicious site to make authenticated requests. Always validate reflected origins against a whitelist.

Privacy & Security

All CORS header validation is performed entirely in your browser using JavaScript. The CORS headers you paste are never sent to any server, never stored in any database, and never leave your device. This ensures complete privacy when checking headers from any server, including internal APIs.

The tool works completely offline after the initial page load. No data is logged, no analytics track your queries, and no server processes your input. Your security testing stays completely private.

Related Tools

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HTTP Security Headers Checker

Check all security-related HTTP headers for any URL: HSTS, CSP, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, and CORS.

Server Header Analyzer

Analyze any Server HTTP header to identify web server software, detect exposed version numbers, and get security recommendations.

Content Security Policy Validator

Validate any CSP header against security best practices. Check for missing directives, unsafe-inline, unsafe-eval, and wildcard sources.

Frequently Asked Questions About Access-Control-Allow-Origin Checker

Access-Control-Allow-Origin (ACAO) is an HTTP response header that specifies which origins are permitted to read the response of a cross-origin request. It is part of the CORS (Cross-Origin Resource Sharing) specification. The header can contain a specific origin (e.g., "https://example.com"), a wildcard ("*") to allow all origins, or "null" for special cases. Browsers enforce this header to prevent unauthorized cross-origin data access.

Using Access-Control-Allow-Origin: * together with Access-Control-Allow-Credentials: true creates a configuration that is explicitly forbidden by the CORS specification. Browsers will reject any request that combines a wildcard origin with credentials. This is a critical security measure because allowing any origin to make credentialed requests would expose user session data to every website. To fix this, replace the wildcard with a specific origin.

CORS (Cross-Origin Resource Sharing) is a browser security mechanism that controls how web pages from one origin can request resources from a different origin. Without CORS, malicious websites could make requests to other sites and read the responses, potentially stealing sensitive data. CORS headers tell the browser which cross-origin requests are legitimate. Proper CORS configuration is essential for web security.

Absolutely. All CORS header validation is performed entirely in your browser using client-side JavaScript. The headers you paste are never sent to any server, never stored in any database, and never leave your device. The tool works completely offline after the initial page load. This means you can safely check headers from any server, including internal APIs and sensitive configurations.

The security grade (A+ to F) reflects the overall security posture of your CORS configuration. A+ (95-100) means excellent security with no issues. A (85-94) means good with minor informational findings. B (75-84) means acceptable with some warnings. C (60-74) means significant gaps need addressing. D (40-59) means poor security with serious issues. F (0-39) means critical security vulnerabilities that need immediate attention.

This tool checks CORS headers against common security misconfigurations and best practices. It detects wildcard origins with credentials, null origins, invalid origin URLs, trailing slashes, duplicate methods, and overly permissive header lists. However, it does not perform runtime testing - it validates the header values themselves. For complete CORS testing, consider also testing with an actual browser-based CORS test tool.

A CORS error indicates a configuration that will cause browsers to block requests or that creates a security vulnerability. For example, using wildcard with credentials is an error because browsers will reject it. A CORS warning indicates a configuration that, while not blocking, is not ideal - such as using a wildcard origin for an endpoint that serves user-specific data. Warnings should be reviewed and addressed but may not cause immediate failures.

Yes, this tool is 100% free to use with no signup required, no usage limits, and no hidden charges. Check as many CORS configurations as you need - there are no restrictions on the number of headers you can validate. No registration, no API keys, and no premium features. All processing happens locally in your browser.