What is CORS and Why Does it Matter?

Cross-Origin Resource Sharing (CORS) is a security mechanism built into web browsers that controls how web pages in one domain can request and interact with resources hosted on another domain. Without proper CORS configuration, your website or API may be vulnerable to various attacks, including data theft, unauthorized actions, and sensitive information exposure.

CORS misconfigurations are among the most common security issues in modern web applications, particularly because:

They're easy to introduce when developers are trying to make APIs work across domains
The consequences aren't immediately obvious during development
Testing for CORS issues requires specific knowledge and tools
Improper configurations can lead to serious security vulnerabilities

Our CORS Tester helps you identify potential CORS-related security issues in your web applications and APIs before attackers can exploit them.

Common CORS Vulnerabilities

CORS misconfigurations can lead to several types of security vulnerabilities:

1. Overly Permissive Access-Control-Allow-Origin

When a server responds with Access-Control-Allow-Origin: * or dynamically reflects the Origin header without validation, any website can access your API's resources with the user's credentials. This could allow malicious websites to steal sensitive data or perform actions on behalf of your users.

2. Improper Credential Handling

If Access-Control-Allow-Credentials: true is combined with a wildcard or insufficiently validated Origin, attackers can make authenticated requests to your API from their malicious domains, potentially accessing or modifying users' private data.

3. Insecure Pre-flight Response Handling

When servers don't properly validate the pre-flight OPTIONS requests or expose too many HTTP methods and headers, attackers can perform actions that should be restricted, potentially bypassing security controls.

4. Null Origin Acceptance

Some servers mistakenly accept requests with Origin: null, which can be spoofed by attackers using various techniques, including sandboxed iframes or local HTML files.

Our CORS Tester checks for all these vulnerabilities and more, helping you ensure your web applications implement CORS securely.

How Our CORS Tester Works

The BrowseSafe CORS Tester performs a comprehensive analysis of your website or API's CORS configuration:

Endpoint Analysis: Tests specific API endpoints for CORS-related headers and behavior
Origin Validation Testing: Checks how your server responds to various Origin headers, including wildcards, null values, and malformed origins
Credential Handling Verification: Examines whether sensitive endpoints properly restrict cross-origin requests with credentials
Pre-flight Request Testing: Analyzes how your server handles OPTIONS requests and what methods and headers it exposes
Header Reflection Analysis: Detects if your server unsafely reflects Origin headers without proper validation
Security Recommendation Generation: Provides specific recommendations to fix any identified issues

The tool is designed to be non-invasive and doesn't perform any actions that could modify data or affect your application's functionality. It only analyzes the CORS-related responses from your server.

Secure CORS Configuration Best Practices

Specific Origin Allowance: Always specify exact domains in Access-Control-Allow-Origin instead of using wildcards
Careful Credential Handling: Never use both Access-Control-Allow-Credentials: true and Access-Control-Allow-Origin: *
Minimal Method and Header Exposure: Only expose necessary HTTP methods and headers in Access-Control-Allow-Methods and Access-Control-Allow-Headers
Appropriate Caching: Set reasonable values for Access-Control-Max-Age to reduce pre-flight request overhead
Reject Null Origins: Ensure your server doesn't accept Origin: null for sensitive operations
Use HTTPS: Always serve your API over HTTPS to protect against network eavesdropping
Layer Defenses: Don't rely solely on CORS for security—implement proper authentication, authorization, and CSRF protection

Following these practices will help you build a secure CORS configuration that properly balances functionality and security.

When to Use Our CORS Tester

You should test your application for CORS misconfigurations:

During the development of new APIs or web services
After making changes to your API's CORS configuration
Before launching a new web application
As part of regular security assessments
When integrating with third-party services
If you suspect your application may have been compromised

Regular testing helps ensure that your application remains secure against evolving threats and that development changes don't inadvertently introduce vulnerabilities.

Fixing CORS Vulnerabilities

If our tester identifies CORS issues in your application, here are general steps to fix them:

For Web Servers

Configure your web server to send the appropriate CORS headers:

Apache (.htaccess)

<IfModule mod_headers.c>
  Header set Access-Control-Allow-Origin "https://trusted-domain.com"
  Header set Access-Control-Allow-Methods "GET, POST, OPTIONS"
  Header set Access-Control-Allow-Headers "Content-Type, Authorization"
  Header set Access-Control-Allow-Credentials "true"
</IfModule>

Nginx

location / {
  if ($request_method = 'OPTIONS') {
    add_header 'Access-Control-Allow-Origin' 'https://trusted-domain.com';
    add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
    add_header 'Access-Control-Allow-Headers' 'Content-Type, Authorization';
    add_header 'Access-Control-Allow-Credentials' 'true';
    add_header 'Content-Length' 0;
    add_header 'Content-Type' 'text/plain charset=UTF-8';
    return 204;
  }
  add_header 'Access-Control-Allow-Origin' 'https://trusted-domain.com';
  add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
  add_header 'Access-Control-Allow-Headers' 'Content-Type, Authorization';
  add_header 'Access-Control-Allow-Credentials' 'true';
}

For Application Frameworks

Many frameworks provide middleware or configuration options for handling CORS:

Express.js (Node.js)

const cors = require('cors');
app.use(cors({
  origin: 'https://trusted-domain.com',
  methods: ['GET', 'POST'],
  allowedHeaders: ['Content-Type', 'Authorization'],
  credentials: true
}));

Spring Boot (Java)

@Configuration
public class WebConfig implements WebMvcConfigurer {
  @Override
  public void addCorsMappings(CorsRegistry registry) {
    registry.addMapping("/**")
      .allowedOrigins("https://trusted-domain.com")
      .allowedMethods("GET", "POST")
      .allowedHeaders("Content-Type", "Authorization")
      .allowCredentials(true);
  }
}

Always remember to replace "https://trusted-domain.com" with your actual trusted domains, and only include the HTTP methods and headers that your application legitimately needs.

CORS Misconfiguration Tester

Identify and Fix Cross-Origin Resource Sharing Vulnerabilities