But, in some situations, for example in web development or app testing, you may need to test something in cross domains. If you don't want any domain-based restrictions (the most common scenario), copy this JSON to a file named cors.json: You will get a warning banner in Chrome notifying about reduces security, because that is actually what you have here. This header needs to be part of the server's response, it does not need to be part of the client's request.Specifically what happens is before the client makes the We have to allow CORS, placing Access-Control-Allow-Origin: in header of request may not work. 7,560. For detecting purpose, we use only our very fast server-side API. Added. 408. This can only happen if your custom JS/CSS was loaded by the extension as a "content script", not a normal