CORS, or Cross-Origin Resource Sharing, is a security feature implemented by browsers to control how resources are requested from one domain by a web application hosted on a different domain. It’s essentially a way to allow or block requests that originate from different origins, which helps prevent malicious websites from accessing restricted resources on other domains.
How CORS Works
When a web application requests resources from a different origin (domain, protocol, or port) than its own, the browser sends an HTTP preflight request (an OPTIONS request) to the server. This request asks for permission to access the resource with specific methods (e.g., GET, POST) and headers. If the server allows this request, it responds with specific CORS headers, such as:
- Access-Control-Allow-Origin: Specifies which domains are permitted to access the resource.
- Access-Control-Allow-Methods: Specifies which HTTP methods are allowed for cross-origin requests.
- Access-Control-Allow-Headers: Specifies which headers can be used when making the request.
If the server permits the cross-origin request, the browser proceeds with the actual request. If not, the browser blocks it.
Why We Should Use CORS
Security: CORS helps prevent unauthorized access to resources. Without it, websites could make unauthorized requests to sensitive resources (like APIs) on behalf of a user without their consent.
Controlled Sharing: It allows developers to specify exactly who can access resources, what methods they can use, and what headers are acceptable, giving fine-grained control over resource access.
API Usage: When building APIs that serve multiple frontends (such as web apps or mobile apps), CORS is essential to allow these applications to interact securely with the API while ensuring access restrictions are in place.
Overall, CORS is a key part of web security and interoperability, ensuring that cross-origin resource requests are made securely and with control.
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