Cross Origin Resource Sharing

Cross-Origin Resource Sharing (CORS)

Definition:

CORS is a security feature implemented by web browsers to control the requests initiated from scripts to a domain different from the one where the script resides. It mitigates the risk associated with cross-origin HTTP requests and allows servers to specify who can access its assets and which HTTP methods can be used when accessing those assets.

Key Points:

  • Historically, web browsers prevented web pages from making requests to a different domain than the one that served the web page due to the "same-origin policy."

  • CORS is a mechanism that allows many requests (especially AJAX requests) to cross domain boundaries.

  • CORS introduces a set of headers that allow the server to specify which sites or origins are permitted to read its data.

How CORS Works:

  1. Simple Requests: For certain types of requests, called "simple requests," the browser sends the request directly. However, it inspects the CORS headers in the response to decide if the response data should be exposed to the page.

  2. Preflighted Requests: For more complicated requests (e.g., requests that use non-standard headers or HTTP verbs other than GET, HEAD, or POST), the browser first sends a "preflight" request using the OPTIONS method to the target domain to check whether the actual request will be allowed.

Example Headers:

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

  • Access-Control-Allow-Methods: Indicates which HTTP methods are allowed on the resource.

  • Access-Control-Allow-Headers: Indicates which HTTP headers can be used when making an actual request.

  • Access-Control-Max-Age: Specifies how long the results of a preflight request can be cached.

Attack Scenario (Misconfigured CORS):

If a website misconfigures its CORS settings (for example, by setting Access-Control-Allow-Origin to *, which means any origin), it can expose its users to potential risks. An attacker could create a malicious website that makes requests to the targeted website and reads the responses, potentially accessing private user data or performing actions on behalf of the user on the targeted site.

Mitigation:

  1. Restrict Origins: Avoid setting Access-Control-Allow-Origin to * for sensitive data. Instead, specify the exact origins that should be allowed.

  2. Avoid Credentials: If possible, avoid using Access-Control-Allow-Credentials: true. If you have to use it, ensure the allowed origins are explicitly defined and not set to *.

  3. Limit Exposed Headers: Use the Access-Control-Expose-Headers to limit which response headers are exposed to the invoking site.

  4. Be Careful with Allowed Methods: Only allow necessary HTTP methods. For example, if your resource doesn't need DELETE, don't include it in Access-Control-Allow-Methods.

  5. Regular Audits: Conduct regular security audits to check for misconfigurations in CORS settings.

Detection:

  1. Manual Testing:

    • Check for the presence of CORS headers in HTTP responses. Tools like browser developer tools or curl can be used to inspect headers.

  2. Automated Scanning:

    • Security scanning tools like OWASP ZAP or Burp Suite can be configured to detect misconfigured CORS headers.

Conclusion:

CORS is a pivotal feature that enables interactive and dynamic web applications by allowing cross-origin requests. However, it also introduces potential risks if not configured correctly. Proper understanding, correct implementation, and regular checks are crucial to ensure that CORS settings protect applications and their users from cross-origin attacks.

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