It seems like the malicious site can steal information.īut in reality, it's usually not a problem. With Access-Control-Allow-Origin: *, evil.example is allowed to read responses from other.example/api. Since that is a cross-origin request, other.example/api sends back an Access-Control-Allow-Origin header. Let's also follow the example above and assume that there is a site.example which is intended to communicate with other.example/api.

The site has some javascript that runs a fetch to other.example/api on your behalf. Let's say you are a user visiting a malicious third-party website at evil.example. This sounds insecure, but let's see the implications! What * allows? In this case, every website can send requests to the target and read the response. This is usually what API developers do when faced with this error. The special value * means 'all origins' which essentially disables this security feature. The Access-Control-Allow-Origin specifies the allowed origin that can make cross-origin requests. Has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on Without an Access-Control-Allow-Origin header in the response, the browser throws an exception: Access to XMLHttpRequest at '' from origin '' This is a security feature as it protects the user by not letting random websites fetch data from sites he is logged in. The two important points are that the target server must allow the operation and the client's browser enforces it. In that case, the server must indicate that it allows the cross-origin operation otherwise the browser will reject the request.

Want to learn AWS serverless development? Click here CORS headersĬORS headers come into play when a client makes a cross-origin request.

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