CORS Checker

API security tool • 2026 standards

CORS Headers:

Check CORS

Access-Control-Allow-Origin: *

Access-Control-Allow-Methods: GET, POST, PUT, DELETE

Access-Control-Allow-Headers: Content-Type, Authorization

Access-Control-Allow-Credentials: true

Cross-Origin Resource Sharing (CORS) is a security feature that allows web browsers to make requests to different origins. It controls which resources can be accessed from other domains.

Best Practices:

  • Specify exact origins instead of wildcard (*)
  • Limit allowed methods to what's necessary
  • Validate and sanitize request headers
  • Use credentials cautiously
  • Implement preflight caching
  • Monitor CORS-related errors

Impact: Proper CORS configuration improves API security, prevents unauthorized cross-origin access, and enables secure API integration.

CORS Configuration

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CORS Analysis

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Allowed Origins
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Allowed Methods
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Allowed Headers
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Status
Access-Control-Allow-Origin:
*
Access-Control-Allow-Methods:
GET, POST, PUT, DELETE
Access-Control-Allow-Headers:
Content-Type, Authorization
Access-Control-Allow-Credentials:
true
CORS Policy Analysis
Configure your CORS settings above to see analysis results. The CORS checker will validate your cross-origin resource sharing policy and provide security recommendations.

Test Results

Run a test to see results
CORS Configuration Examples
// Secure CORS configuration app.use(cors({ origin: ['https://myapp.com', 'https://admin.myapp.com'], methods: ['GET', 'POST', 'PUT', 'DELETE'], allowedHeaders: ['Content-Type', 'Authorization'], credentials: true }));
// Server-side response headers Access-Control-Allow-Origin: https://myapp.com Access-Control-Allow-Methods: GET, POST Access-Control-Allow-Headers: Content-Type Access-Control-Allow-Credentials: true

CORS Fundamentals

CORS Concepts

Cross-Origin Resource Sharing (CORS) is a security feature that allows web browsers to make requests to different origins. It controls which resources can be accessed from other domains.

CORS Headers

Access-Control-Allow-Origin: Specifies which origins can access the resource

Access-Control-Allow-Methods: Lists HTTP methods allowed

Access-Control-Allow-Headers: Lists headers allowed in requests

Access-Control-Allow-Credentials: Allows credentials to be sent

These headers control cross-origin access to resources.

Key Rules:
  • Specify exact origins instead of wildcard
  • Limit allowed methods to what's necessary
  • Validate and sanitize request headers
  • Use credentials cautiously
  • Implement preflight caching
  • Monitor CORS-related errors

Security Impact

Security Benefits

Proper CORS configuration improves API security, prevents unauthorized cross-origin access, enables secure API integration, protects against cross-site request forgery, and maintains data integrity across domains.

Common Security Issues
  1. Overly permissive origins
  2. Unsafe methods allowed
  3. Unrestricted headers
  4. Credentials misconfiguration
  5. Preflight bypass attempts
  6. Origin spoofing
Common Issues:
  • Wildcard origins (*)
  • Unsafe header access
  • Missing preflight validation
  • Insufficient error handling

CORS Security Learning Quiz

Question 1: Multiple Choice - CORS Purpose

What is the primary purpose of CORS (Cross-Origin Resource Sharing)?

Solution:

The answer is B) To control cross-origin access to resources. CORS is a security feature that allows web browsers to make requests to different origins. It controls which resources can be accessed from other domains, preventing unauthorized cross-origin access.

Pedagogical Explanation:

CORS solves the "same-origin policy" restriction by allowing controlled exceptions. Without CORS, web browsers would block requests to different domains for security reasons. CORS provides a secure way to allow cross-origin requests when appropriate.

Key Definitions:

CORS: Cross-Origin Resource Sharing

Same-Origin Policy: Browser security restriction

Cross-Origin: Different domain, protocol, or port

Important Rules:

• Controls cross-origin access

• Browser security feature

• Server must allow origins

• Prevents unauthorized access

Tips & Tricks:

• Specify exact origins

• Limit allowed methods

• Validate headers

Common Mistakes:

• Using wildcard origins

• Allowing unsafe methods

• Not validating requests

Question 2: CORS Security

Which of the following CORS configurations is the most secure?

Solution:

The answer is B) Access-Control-Allow-Origin: https://trusted.com. This configuration explicitly allows only the trusted domain to access the resource, providing the best security while allowing legitimate cross-origin access.

Pedagogical Explanation:

Wildcard origins (*) are the least secure as they allow any domain to access the resource. Specifying exact origins provides the best security by limiting access to only authorized domains. This follows the principle of least privilege.

Key Definitions:

Wildcard Origin: Allows any domain (*)

Exact Origin: Specific domain only

Least Privilege: Minimum access necessary

Important Rules:

• Avoid wildcard origins

• Specify exact origins

• Validate origin requests

• Use HTTPS origins

Tips & Tricks:

• Use specific domains

• Validate during runtime

• Consider subdomains

Common Mistakes:

• Using wildcard origins

• Not validating origins

• Trusting user input

Question 3: Word Problem - Preflight Requests

When does a browser make a preflight request for a CORS request? What headers are required for the preflight response?

Solution:

A browser makes a preflight request when the request is not "simple" - typically when:

  • Using methods other than GET, POST, or HEAD
  • Sending custom headers (other than simple headers)
  • Using content types other than application/x-www-form-urlencoded, multipart/form-data, or text/plain

Required preflight response headers:

  • Access-Control-Allow-Origin: Specifies allowed origins
  • Access-Control-Allow-Methods: Lists allowed HTTP methods
  • Access-Control-Allow-Headers: Lists allowed request headers
  • Access-Control-Max-Age: Optional, specifies cache duration
Pedagogical Explanation:

Preflight requests are an extra security step where the browser first asks the server if the actual request is safe to make. This prevents potentially dangerous cross-origin requests from being made without server consent.

Key Definitions:

Preflight Request: Initial OPTIONS request

Simple Request: Doesn't require preflight

Complex Request: Requires preflight

Important Rules:

• Preflight for complex requests

• Use OPTIONS method

  • Include required headers

    • • Cache for efficiency

    Tips & Tricks:

    • Cache preflight responses

    • Use simple requests when possible

    • Minimize preflight triggers

    Common Mistakes:

    • Missing preflight headers

    • Not handling OPTIONS

    • Invalid response headers

    Question 4: Application-Based Problem - Credentials

    When using Access-Control-Allow-Credentials: true, what additional requirement must be met for the Access-Control-Allow-Origin header?

    Solution:

    When using Access-Control-Allow-Credentials: true, the Access-Control-Allow-Origin header cannot be set to a wildcard (*). It must specify an exact origin. This is a security requirement to prevent credential leakage to unauthorized origins.

    Example of correct configuration:

    Access-Control-Allow-Origin: https://trusted.com
Access-Control-Allow-Credentials: true

    Example of incorrect configuration:

    Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

    This combination is forbidden by the CORS specification.

    Pedagogical Explanation:

    This restriction exists because allowing credentials with wildcard origins would enable malicious sites to make authenticated requests on behalf of users, potentially leading to session hijacking or other security breaches.

    Key Definitions:

    Credentials: Cookies, authorization headers

    Security Requirement: Prevent credential leakage

    Wildcard Restriction: Cannot combine with credentials

    Important Rules:

    • No wildcard with credentials

    • Exact origin required

    • Security by design

    • Prevents session hijacking

    Tips & Tricks:

    • Use specific origins

    • Validate origin carefully

    • Consider alternative solutions

    Common Mistakes:

    • Using wildcard with credentials

    • Not understanding restriction

    • Incorrect header combinations

    Question 5: Multiple Choice - CORS Headers

    Which of the following headers is NOT a standard CORS response header?

    Solution:

    The answer is C) Access-Control-Allow-Resources. This is not a standard CORS header. The standard CORS response headers are: Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, Access-Control-Allow-Credentials, Access-Control-Expose-Headers, and Access-Control-Max-Age.

    Pedagogical Explanation:

    It's important to know the standard CORS headers to properly configure your server. Non-standard headers won't be recognized by browsers for CORS validation and won't provide the intended cross-origin access control.

    Key Definitions:

    Standard Headers: Official CORS specification

    Non-Standard: Not recognized by browsers

    Specification: W3C CORS standards

    Important Rules:

    • Use standard headers only

    • Follow W3C specification

    • Non-standard ignored

    • Validate header names

    Tips & Tricks:

    • Refer to official specs

    • Use established libraries

    • Test header names

    Common Mistakes:

    • Using non-standard headers

    • Typo in header names

    • Creating custom headers

    cors-checker

    FAQ

    Q: What's the difference between simple and complex CORS requests?

    A: The difference between simple and complex CORS requests determines whether a preflight request is made:

    Simple Requests:

    • Uses GET, POST, or HEAD methods
    • Only uses simple headers (Accept, Accept-Language, Content-Language, Content-Type with values application/x-www-form-urlencoded, multipart/form-data, or text/plain)
    • No custom headers
    • No preflight request needed

    Complex Requests:

    • Uses methods other than GET/POST/HEAD (PUT, DELETE, PATCH, etc.)
    • Uses custom headers
    • Uses content types other than the simple ones
    • Requires preflight OPTIONS request

    Complex requests require a preflight check to ensure the server allows the cross-origin request.

    Q: How should I handle CORS in production environments?

    A: Best practices for CORS in production:

    Server-Side:

    • Specify exact origins (never use wildcard *)
    • Limit allowed methods to only what's necessary
    • Validate and sanitize request headers
    • Implement proper error handling
    • Use HTTPS for all CORS-enabled endpoints

    Security Measures:

    • Don't allow credentials with wildcard origins
    • Implement rate limiting for CORS requests
    • Log CORS-related activity for monitoring
    • Regularly audit CORS configurations
    • Use CORS libraries with built-in security

    Monitoring:

    • Track CORS preflight requests
    • Monitor blocked requests
    • Alert on unusual CORS patterns

    Remember to test thoroughly in staging before deploying to production.

    About

    Security Team
    This CORS checker was created
    This calculator was created by our Web & Development Team , may make errors. Consider checking important information. Updated: April 2026.