What is CORS and How to Fix It.
CORS (Cross-Origin Resource Sharing) is a browser security feature that controls which cross-site requests are allowed, preventing unauthorized access between different domains.
When to Use
CORS is used when a web application hosted on one domain needs to request resources (APIs, assets, data) from another domain. It’s common in modern front-end + API architectures.
Example
A React app running on http://localhost:3000 fetching data from https://api.example.com will fail with a CORS error unless the API server allows that origin.
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Why Is It Important
CORS enforces the same-origin policy, which protects users by stopping malicious websites from secretly reading data from another site.
Interview Tips
In interviews, explain CORS clearly and mention that fixing it usually requires adjusting server-side headers like Access-Control-Allow-Origin. Demonstrating both security awareness and practical fixes shows strong engineering maturity.
Trade-offs
Using * to allow all origins is quick but risky—it weakens security. Limiting access to specific origins is safer but requires maintenance when clients change.
Pitfalls
A common mistake is trying to “fix” CORS in the browser or client code—the solution must come from the server. Another is using * with credentials, which won’t work.
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