If we issue a request which is not considered “simple”, for example by setting the Content-Type header to application/json, the browser will try to fulfil a “preflight” request/response cycle with the server before sending the request. The CORS policy contains a definition of “simple requests”. Keep in mind that the CORS policy doesn’t necessarily prevent requests to your backend but only prevents the script accessing the response! This is just another reason for honouring http safe methods. Example flow diagram of a CORS request/response cycle Another option would be to generate an exactly matching response. It is common for servers to reply with a dynamically generated Access-Control-Allow-Origin: * header for allowed origins and omit it otherwise. The browser checks if the origins match and if so, makes the response available to the script. The server then needs to add an Access-Control-Allow-Origin header to the response. For CORS requests the browser adds an additional “Origin” header. Since is outside the same-origin, we call this Cross-Origin Resource Sharing. A webpage might be served under and you need to access. There are still situations where it makes sense to access a resource outside the own origin. You don’t want foo.js to be able to send requests to your webmail provider, twitter account or similar things. Without the SOP in place, a javascript delivered by a malicious server could send requests to an application you are already authenticated. It is a crucial security mechanism preventing the execution of requests by malicious scripts which might try to access data of other webpages. This is implemented by the browsers as the “Same-Origin-Policy (SOP)”. Usually scripts and documents are restricted to only interact with their own origin. The origin consists of a scheme, the hostname and an optional port. A short CORS recapĮvery webpage which is delivered to you through the browser has a specific origin. To understand the change lets have a short recap of CORS and its intentions. via VPN) you might run into problems with Chrome. If however your application is included in an iframe and accessed through your local intranet (e.g. It might not affect you at all if the user facing part of your application is hosted on servers within a public IP range. This changes the browsers CORS mechanism if you try to access resources on a server located within an IP range considered as private. Google is experimenting with the implementation of its W3C Draft “Private Network Access” with Chrome 98.
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