Identification of Cross-Origin Resource Sharing (CORS) Misconfiguration

Cross-Origin Resource Sharing (CORS) is a mechanism that allows web applications running at one origin (domain) to access resources from a server at a different origin. While CORS is essential for modern web applications to function across various domains, its misconfiguration can lead to security vulnerabilities, potentially allowing malicious websites to access sensitive information from another domain. Here’s how a penetration tester might identify CORS misconfiguration:


  1. Overly Permissive Origins: Check if the Access-Control-Allow-Origin header is set to `` (wildcard), allowing any domain to access the resource. This setting is particularly risky for credentials-included requests.
  2. Credentials Inclusion: Verify if Access-Control-Allow-Credentials is set to true while the Access-Control-Allow-Origin header is too permissive. This combination can lead to the exposure of sensitive information to unauthorized origins.
  3. Excessive HTTP Methods: Review the Access-Control-Allow-Methods header to see if it permits more HTTP methods than necessary, potentially opening up the server to unwanted actions via cross-origin requests.
  4. Unrestricted Headers: The Access-Control-Allow-Headers header should be scrutinized to ensure it doesn't allow more headers than required, which could expose sensitive information or control mechanisms.
  5. Preflight Requests: Inspect how preflight OPTIONS requests are handled. Misconfigured responses to preflight requests can lead to improper CORS behavior, allowing restricted cross-origin calls.
  6. Specific Origins with Credentials: Check if the CORS policy allows specific origins with credentials. Be cautious if the allowed origin seems to be dynamically reflected based on the Origin request header, which can be manipulated.

Testing Techniques

  • Origin Reflection: Alter the Origin header in requests to see if the server reflects the origin in the Access-Control-Allow-Origin header, which might indicate a misconfiguration.
  • Testing with Browser Tools: Use browser development tools or extensions to modify CORS headers and observe the application's behavior, identifying potential misconfigurations.
  • Automated Scanning: Employ automated tools and scripts that test for common CORS misconfigurations by sending various cross-origin requests and analyzing the responses.


  1. Validate Origins: Instead of a wildcard, validate the Origin header against a whitelist of allowed domains, especially when credentials are included.
  2. Limit Methods and Headers: Restrict methods and headers to those necessary for the application's functionality.
  3. Secure Configuration: Ensure the CORS policy is configured securely in all environments, including development, testing, and production.
  4. Regular Auditing: Regularly review and audit CORS policies to adapt to changes in the application and prevent misconfigurations.
  5. Education and Awareness: Ensure that developers are aware of CORS' security implications and understand how to configure it correctly.

By identifying and addressing CORS misconfigurations, penetration testers can help organizations prevent potential vulnerabilities that could expose sensitive information or allow unauthorized actions on behalf of users.