CORS Tester

Tool Overview

A CORS tester checks if cross-origin requests are allowed between websites. CORS stands for Cross-Origin Resource Sharing. It is a browser security feature that controls which websites can access resources from your API or server.

Modern web applications often need to fetch data from APIs on different domains. Browsers block these requests by default for security. Servers must explicitly allow cross-origin requests using special headers. The problem is that CORS configuration can be complex. Missing headers, wrong values, or credential conflicts can block legitimate requests. Without testing, you cannot tell if CORS is configured correctly until users report errors.

This tool is for API developers, frontend developers, and system administrators. Beginners can use it to understand CORS behavior. Technical users can debug CORS issues quickly. Professionals can verify API configurations before deployment. A related operation involves checking HTTP status codes as part of a similar workflow.

Background & Concept Explanation

CORS is a browser security mechanism. It prevents websites from making requests to other domains without permission. This stops malicious sites from stealing data or making unauthorized API calls. When a website tries to fetch data from another domain, the browser checks CORS headers in the response.

CORS works through special HTTP headers. The most important header is Access-Control-Allow-Origin. It tells the browser which origins are allowed to access the resource. Origins include the protocol, domain, and port. For example, https://example.com:443 is one origin. http://example.com:80 is a different origin. For adjacent tasks, checking HTTP headers addresses a complementary step.

Some requests trigger a preflight check. The browser sends an OPTIONS request first to ask permission. The server responds with CORS headers. If the preflight succeeds, the browser sends the actual request. Simple requests like GET with standard headers skip preflight. Complex requests like PUT or requests with custom headers require preflight.

People struggle with CORS for several reasons. They do not understand when preflight is required. They forget to configure CORS headers on their server. They use wildcard origins with credentials, which browsers block. They do not know which headers to set. They test from the same origin, which never triggers CORS checks. When working with related formats, testing robots.txt files can be a useful part of the process.

This tool solves these problems by performing real browser-based tests. It makes actual HTTP requests from your browser. It shows whether requests succeed or fail. It explains why requests are blocked. It analyzes CORS headers and identifies issues. It provides code snippets to fix problems.

Key Features

• Live browser testing: Makes real HTTP requests from your browser to test CORS behavior. This matters because it shows actual browser behavior, not simulated results. You see exactly what happens when your application makes requests.
• Manual header analysis: Paste HTTP response headers directly for analysis without making requests. This matters because you can test headers from logs, documentation, or other sources without network access.
• HTTP method selection: Choose from GET, POST, PUT, DELETE, PATCH, OPTIONS, or HEAD methods. This matters because different methods may have different CORS requirements, and some trigger preflight checks.
• Origin detection: Automatically detects and displays your current origin. This matters because CORS checks depend on the requesting origin, and you need to know what origin the server sees.
• Preflight detection: Identifies when preflight OPTIONS requests are required. This matters because preflight failures can block requests even if the actual request would succeed.
• Verdict status: Shows clear verdicts: Allowed when requests succeed, Blocked when CORS prevents access, Error when network issues occur. This matters because it gives you immediate feedback on CORS configuration.
• Detailed explanations: Provides plain English reasons for each verdict. Lists specific details about what caused the result. This matters because it helps you understand why requests succeed or fail.
• CORS header analysis: Extracts and displays all Access-Control headers from responses. Shows header values and explains their meaning. This matters because you can see exactly which headers the server sends.
• Header validation: Checks for common CORS configuration errors like credential conflicts with wildcard origins. Validates origin matching and method permissions. This matters because it catches mistakes before they cause problems.
• Response time measurement: Measures how long requests take to complete. This matters because slow responses can indicate server issues or network problems.
• Request timeout handling: Sets a 30-second timeout for requests. Handles timeout errors gracefully with clear messages. This matters because it prevents hanging requests and provides feedback when servers are slow.
• AI-powered fix suggestions: Optional feature generates server-side code to fix CORS issues. Provides code snippets for common web frameworks and servers. This matters because it gives you actionable solutions instead of just identifying problems.
• Input validation: Validates URLs and header input before processing. Maximum URL length is 2048 characters. Maximum manual headers input is 50KB. This matters because it prevents errors and keeps the tool responsive.

Common Use Cases

Use this tool in these situations: In some workflows, testing webhooks is a relevant follow-up operation.

• Debugging API access issues: If your frontend application cannot fetch data from an API, test CORS to see if headers are blocking requests.
• Verifying CORS configuration: After setting up CORS headers on your server, test them to ensure they work correctly for your frontend origin.
• Testing different HTTP methods: Check if your API allows POST, PUT, DELETE, or other methods from cross-origin requests.
• Learning CORS behavior: Understand when preflight requests occur and how CORS headers control access by testing different scenarios.
• Comparing API configurations: Test APIs from different providers to see how they configure CORS and learn best practices.
• Pre-deployment verification: Before deploying API changes, test CORS to ensure existing frontend applications continue to work.
• Analyzing header logs: Paste headers from server logs or browser dev tools to analyze CORS configuration without making new requests.
• Fixing credential issues: Test APIs that require cookies or authentication headers to verify credential handling is configured correctly.

How to Use This Tool (Step-by-Step)

1. Open the tool and choose your testing mode. Click Live Test to make real browser requests, or click Manual Headers to analyze headers you paste.
2. If using Live Test, select an HTTP method from the dropdown. Choose GET for simple requests, POST for form submissions, PUT or DELETE for updates, or other methods as needed.
3. Enter the API URL you want to test. You can include or omit the protocol. The tool will add https:// automatically if needed. Maximum length is 2048 characters.
4. Check the origin display below the input. It shows your current origin. This is the origin the server will see in the request.
5. Click Analyze CORS to start the test. The tool makes an HTTP request from your browser and checks the response.
6. Wait for the verdict. The tool shows Allowed if the request succeeds, Blocked if CORS prevents access, or Error if network issues occur.
7. Read the verdict explanation. It explains why the request succeeded or failed. Review the details list for specific information about CORS headers and configuration.
8. Check the Detected CORS Headers section if headers were found. It shows all Access-Control headers with their values. Use this to see exactly what headers the server sends.
9. If the request was blocked, use the AI Fix Assistant. Click Generate Fix to get server-side code suggestions. Review the code and adapt it for your server configuration.
10. If using Manual Headers mode, paste HTTP response headers in the text area. Headers should be in the format "Header-Name: value" with one header per line. Maximum size is 50KB. Click Analyze CORS to process them.
11. After fixing CORS issues on your server, run a new test to verify the changes work correctly. Compare the new verdict with the previous one.

Calculations & Logic

This tool performs request testing and header analysis, not numeric calculations.

The live test process works by making a real browser fetch request. The tool uses the Fetch API with CORS mode enabled. It measures request time from start to completion. If the request succeeds, the browser allowed it, meaning CORS headers were correct. If the request fails with a network error, the browser blocked it, meaning CORS headers were missing or incorrect. For related processing needs, testing API endpoints handles a complementary task.

Preflight detection checks if a preflight OPTIONS request would be required. Simple requests like GET, HEAD, or POST with standard headers do not require preflight. Requests with custom headers or non-simple methods require preflight. The tool determines this based on the HTTP method and headers used.

Manual header analysis parses header text line by line. It splits each line at the colon to separate header names from values. It normalizes header names to lowercase for consistent matching. It extracts CORS-specific headers like Access-Control-Allow-Origin, Access-Control-Allow-Methods, and Access-Control-Allow-Headers.

Verdict determination follows browser CORS logic. If Access-Control-Allow-Origin is missing, the verdict is Blocked. If the origin does not match the allowed origin and it is not a wildcard, the verdict is Blocked. If Access-Control-Allow-Credentials is true and Access-Control-Allow-Origin is a wildcard, the verdict is Blocked due to credential conflict. Otherwise, the verdict is Allowed.

Header validation checks for common errors. It verifies origin matching is exact when not using wildcards. It detects credential conflicts with wildcard origins. It validates that required headers are present. It checks method and header permissions when provided.

The AI fix generation sends the verdict, reason, details, and URL to a backend service. The service analyzes the CORS issue and generates server-side code suggestions. Results include code snippets for common frameworks like Express, Flask, or Nginx. The code is returned as plain text with explanations.

Reference Tables or Scales

Tips, Limitations & Best Practices

• Test from different origins: CORS only matters for cross-origin requests. If you test from the same origin as the API, CORS checks never occur. Use the tool from a different domain to see real CORS behavior.
• Understand preflight requirements: Simple GET requests may not trigger preflight, but PUT or custom headers will. Test with the actual method and headers your application uses.
• Avoid wildcard with credentials: Browsers block requests when Access-Control-Allow-Origin is * and Access-Control-Allow-Credentials is true. Use specific origins instead.
• Check method permissions: If your request uses PUT or DELETE, ensure Access-Control-Allow-Methods includes these methods. Missing method permissions cause preflight failures.
• Verify header permissions: If your request includes custom headers like Authorization or X-API-Key, ensure Access-Control-Allow-Headers includes them. Missing header permissions cause preflight failures.
• Use manual headers for debugging: If live tests fail, copy headers from browser dev tools and use manual analysis. This helps identify which headers are missing or incorrect.
• Test after server changes: After modifying CORS configuration on your server, test again to verify changes work. Clear browser cache if preflight caching is involved.
• Review AI suggestions carefully: AI fix suggestions are templates. Adapt them to your specific server setup and security requirements. Test in staging before production.
• Limitations of the tool: The tool can only test publicly accessible URLs. It cannot test APIs behind authentication unless credentials are handled by the browser. Some servers may block automated requests. The tool tests from your current browser origin, which may differ from your application's origin.
• Network dependencies: Live testing requires network access to the target URL. If the server is down or blocking requests, tests will fail. Response times depend on network conditions.
• Browser differences: Different browsers may handle CORS slightly differently. Test in multiple browsers if you encounter inconsistent behavior.
• Preflight caching: Browsers cache preflight responses based on Access-Control-Max-Age. If you change CORS configuration, you may need to wait for the cache to expire or clear it manually.
• AI analysis requires internet: The AI fix feature calls a backend service. It will not work offline. If generation fails, check your connection and try again.