Table of Contents:

• The Role and Purpose of CORS
• Is Enabling CORS Inherently Risky?
• Security Risks Associated with Poorly Configured CORS
• Overly Permissive Access Control
• Insufficient Method and Header Restrictions
• Improper Input Validation
• Best Practices for Mitigating CORS Risks
• FAQ

Is enabling CORS a security risk

Did you know that your website could be vulnerable to attacks even if you’ve enabled CORS? Cross-Origin Resource Sharing (CORS) lets web apps request resources from different origins, but are you sure it’s set up properly? The following explains the risks, along with guidelines to keep your application secure.

The Role and Purpose of CORS

CORS was created to overcome the restrictions of the Same-Origin Policy (SOP). This SOP is a vital security feature in browsers. It stops scripts from a malicious website from accessing your sensitive data on another site without your consent. That is important, fetching information from public APIs or loading files from CDNs are examples of valid use cases that require controlled relaxation of these restrictions.

CORS gives servers a standard way to indicate which origins they allow to access their files using HTTP headers such as Access-Control-Allow-Origin.

Is Enabling CORS Inherently Risky?

It isn’t enabling CORS itself that produces a security problem. Instead, it’s how you set it up. If configured correctly, CORS only allows reliable origins to access sensitive data, keeping strong protection against unauthorized cross-origin requests.
However, mistakes in setup or overly generous settings can weaken the protections SOP offers. They might expose your applications to several threats.

Security Risks Associated with Poorly Configured CORS

These are a few risks associated with incorrect setup:

• Overly permissive access control
• Insufficient method and header restrictions
• Improper input validation

Overly Permissive Access Control

A frequent mistake is setting the Access-Control-Allow-Origin header value too openly. A developer may use an asterisk (*) rather than listing exact, trusted origins. Such a setup allows any domain on the internet to make requests and potentially access private data intended for specific clients only. Such lenient policies may help attacks like Cross-Site Request Forgery (CSRF) if authentication checks elsewhere in the app stack are insufficient.

Insufficient Method and Header Restrictions

Another common oversight is not limiting the HTTP methods (like GET or POST) or headers that are allowed. Server responses, through headers like Access-Control-Allow-Methods and Access-Control-Allow-Headers, should include such limits. If you use default settings without consideration, attackers may exploit this oversight by sending unauthorized requests that bypass intended controls.

Improper Input Validation

Before responding with CORS directives, servers need to carefully check incoming request headers. If not, attackers may alter these values through carefully created requests. They create harmful origin strings or modified header fields, designing them to look legitimate, yet grant them unauthorized permissions.

Best Practices for Mitigating CORS Risks

To set up CORS properly:

• Be specific about origins – Never use * in production.
• Limit HTTP methods – Only allow methods needed by clients.
• Validate headers – Always validate incoming headers.
• Use HTTPS – Always use HTTPS to protect data in transit.

FAQ

What is the Same-Origin Policy?

The Same-Origin Policy is a security feature built into web browsers. It prevents scripts from one origin (domain, protocol, as well as port) from accessing resources from a different origin. CORS is a mechanism to relax this policy in a controlled way.

Why is CORS needed?

CORS is needed to allow web applications to access resources from different domains securely. This is common when using APIs hosted on separate servers or loading resources from CDNs.

What happens if CORS is not configured correctly?

If CORS is not configured correctly, it can leave your application vulnerable to attacks, such as Cross-Site Request Forgery (CSRF). It may allow unauthorized access to sensitive data.

Resources & References:

1. https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS
2. https://developer.mozilla.org/en-US/docs/Web/Security/Practical_implementation_guides/CORS
3. https://konghq.com/blog/learning-center/what-is-cors-cross-origin-resource-sharing
4. https://zerothreat.ai/blog/cors-explained-mitigating-cross-origin-risks
5. https://github.blog/security/application-security/localhost-dangers-cors-and-dns-rebinding/