CORS: Why It Exists, How It Works & How to Fix Common Issues

Short summary

CORS (Cross-Origin Resource Sharing) is a browser-enforced security mechanism that controls which origins can access server resources. Configuration requires matching protocols, domains, and ports; common mistakes include missing origin allowlists and forgetting to allow custom headers like Authorization. The mechanism blocks JavaScript access to responses when origins don't match, not the requests themselves.

• •CORS is a browser security rule enforced at the client, not the backend—both sides must agree via HTTP headers
• •Origins differ if protocol, domain, or port changes; all three must match or CORS restrictions apply
• •Common fixes: allowlist the frontend origin, permit custom headers like Authorization, and understand preflight OPTIONS requests are browser-initiated security checks

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