CORS Preflight & Header Policy Decision Studio

What does the CORS Preflight & Header Policy Decision Studio do?

The CORS Preflight & Header Policy Decision Studio simulates cross-origin HTTP OPTIONS handshakes, validates W3C Fetch Specification CORS header rules, visualizes browser error decision trees, and exports multi-framework server configurations (Express, Nginx, FastAPI, Spring Boot, Istio).

Developers use this utility to diagnose why browsers throw Access-Control-Allow-Origin errors, calculate effective preflight cache durations (Access-Control-Max-Age), and verify Access-Control-Expose-Headers rules for JavaScript client access.

Core CORS Decision Tree Concepts

Cross-Origin Resource Sharing (CORS) is a W3C standard enforced by modern web browsers to prevent unauthorized cross-origin requests.

• Preflight OPTIONS Request: An automatic HTTP OPTIONS request sent by the browser prior to the actual request when custom HTTP methods (e.g. PUT, DELETE), credentials (cookies, Authorization headers), or non-simple headers (e.g. Content-Type: application/json) are used.
• Wildcard Credential Violation: Browsers strictly forbid returning wildcard Access-Control-Allow-Origin: * when credentials mode is include or Access-Control-Allow-Credentials: true.
• Preflight Cache Capping: Browsers enforce maximum upper bounds on Access-Control-Max-Age: Chromium caps cache at 7200s (2 hours), WebKit/Safari caps at 600s (10 minutes), and Firefox allows up to 86400s (24 hours).

How to use the tool?

1. Configure Request Parameters: Select the HTTP Method (POST, PUT, DELETE), Credentials Mode (include, omit), Request Origin (https://app.example.com), and requested headers (Authorization, Content-Type).
2. Configure Server Response Headers: Enter the server's Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, Access-Control-Allow-Credentials, and Access-Control-Max-Age values.
3. Alternatively Paste Raw Dump: Paste a raw HTTP request/response headers dump to auto-populate all input fields.
4. Run Simulation: Click 🚀 Run CORS Simulation to view the interactive 5-step decision tree, status banner, browser cache caps, and 1-click multi-framework config code.

REST API Integration

Blueutils provides a free REST API endpoint (POST https://blueutils.com/api/network/cors-preflight-studio) for programmatic integration.

API Request Parameters

API Request Payload Examples

cURL

curl -X POST https://blueutils.com/api/network/cors-preflight-studio \
  -H "Content-Type: application/json" \
  -d '{
    "origin": "https://app.example.com",
    "method": "POST",
    "requestHeaders": "Authorization, Content-Type",
    "credentialsMode": "include",
    "allowOrigin": "https://app.example.com",
    "allowCredentials": "true",
    "allowMethods": "GET, POST, OPTIONS",
    "allowHeaders": "Authorization, Content-Type"
  }'

Python

import requests

url = "https://blueutils.com/api/network/cors-preflight-studio"
payload = {
    "origin": "https://app.example.com",
    "method": "POST",
    "requestHeaders": "Authorization, Content-Type",
    "credentialsMode": "include",
    "allowOrigin": "https://app.example.com",
    "allowCredentials": "true",
    "allowMethods": "GET, POST, OPTIONS",
    "allowHeaders": "Authorization, Content-Type"
}
headers = {"Content-Type": "application/json"}
response = requests.post(url, json=payload, headers=headers)
print(response.json())

Java

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class Main {
    public static void main(String[] args) throws Exception {
        String jsonPayload = "{\"origin\":\"https://app.example.com\",\"method\":\"POST\",\"allowOrigin\":\"https://app.example.com\",\"allowCredentials\":\"true\"}";
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://blueutils.com/api/network/cors-preflight-studio"))
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(jsonPayload))
            .build();
        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.body());
    }
}

API Response Parameters

API Response Payload Examples

Success Response (HTTP 200 OK)

{
  "isValid": true,
  "isAllowed": true,
  "isPreflightRequired": true,
  "blockReason": null,
  "origin": "https://app.example.com",
  "method": "POST",
  "browserCacheLimits": {
    "chrome": 7200,
    "firefox": 86400,
    "safari": 600
  }
}

Validation Failure Response (HTTP 400 Bad Request)

{
  "isValid": false,
  "error": "Please enter a valid request Origin (e.g., https://example.com)."
}

Rate Limit Exceeded Response (HTTP 429 Too Many Requests)

{
  "error": "API rate limit exceeded. Please wait or contact support@blueutils.com."
}

Why use an API to evaluate CORS headers?

Integrating the CORS Preflight Studio API into CI/CD pipelines, automated testing, or DevOps validation scripts provides significant advantages:

• Automated Security Audit: Programmatically verify that API gateways and cloud load balancers return compliant CORS headers without wildcard credential violations.
• Optimized Token Efficiency for AI Agents: Offload complex CORS state machine evaluation and multi-framework code generation to an external API to minimize token consumption.
• Deterministic Accuracy Without Hallucinations: Language models can occasionally misjudge subtle CORS rules. Delegating processing to a deterministic W3C state machine engine guarantees 100% computational accuracy every time.

Native Usage

How to inspect and debug CORS preflight requests locally using native OS tools:

Windows (CMD / PowerShell)

# PowerShell native OPTIONS preflight request check
$headers = @{
    "Origin" = "https://app.example.com"
    "Access-Control-Request-Method" = "POST"
    "Access-Control-Request-Headers" = "Authorization, Content-Type"
}
$response = Invoke-WebRequest -Uri "https://api.example.com/v1/data" -Method OPTIONS -Headers $headers
$response.Headers | Format-Table -AutoSize

Linux / Unix (cURL OPTIONS Preflight Simulation)

# Send an explicit HTTP OPTIONS preflight request using cURL
curl -i -X OPTIONS https://api.example.com/v1/data \
  -H "Origin: https://app.example.com" \
  -H "Access-Control-Request-Method: POST" \
  -H "Access-Control-Request-Headers: Authorization, Content-Type"

Python (Native urllib CORS Header Check)

import urllib.request

req = urllib.request.Request(
    "https://api.example.com/v1/data",
    headers={
        "Origin": "https://app.example.com",
        "Access-Control-Request-Method": "POST",
        "Access-Control-Request-Headers": "Authorization"
    },
    method="OPTIONS"
)
with urllib.request.urlopen(req) as response:
    print("Access-Control-Allow-Origin:", response.headers.get("Access-Control-Allow-Origin"))
    print("Access-Control-Allow-Credentials:", response.headers.get("Access-Control-Allow-Credentials"))

Java (Native HttpClient CORS Header Inspection)

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class CorsInspector {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.example.com/v1/data"))
            .method("OPTIONS", HttpRequest.BodyPublishers.noBody())
            .header("Origin", "https://app.example.com")
            .header("Access-Control-Request-Method", "POST")
            .header("Access-Control-Request-Headers", "Authorization, Content-Type")
            .build();

        HttpResponse<Void> response = client.send(request, HttpResponse.BodyHandlers.discarding());
        response.headers().map().forEach((key, values) -> {
            if (key.toLowerCase().startsWith("access-control-")) {
                System.out.println(key + ": " + String.join(", ", values));
            }
        });
    }
}