You can’t ask your users to trick their browsers by installing a plugin that applies an header in the frontend. If the frontend domain does not match the value, the browser raises the red flag and blocks the API request with the CORS policy error. Once the browser receives this header information back, it compares the frontend domain with the Access-Control-Allow-Origin value from the server. One: the server can be really strict, and specify that only one origin can access it:Īccess-Control-Allow-Origin: Two: the server can let the gates go wide open, and specify the wildcard value to allow all domains to access its resources: This header contains an Access-Control-Allow-Origin key, to specify which origins can access the server’s resources. Origin: Reacting to this special request, the server sends back a response header. For example, for an app running on localhost:3000, the special request format looks like this: To conduct the same-origin check, the browser accompanies all requests with a special request that sends the domain information receiving server. For example, in the protocol is the host is and the hidden port number is 443 (the port number typically used for https). But really, the origin is the combination of the protocol, host, and port. Above, the origins were simplified to the frontend application and backend server domains. Under the hood, the browser checks if the origins of the web application and the server match. How does the same-origin policy work under the hood? It will stop evil-site and say “Blocked by the same-origin policy. Luckily, in this situation, like a hawk ready to strike, the browser will step in and prevent the malicious code from making an API request like this. Your account has been successfully hacked with a cross-site request forgery attack. Evil-site sends the session cookie, and gains authenticated access to facebook-clone. Since the request is going to the domain, the browser includes the relevant cookies. The evil site also has the ability send a request to /api. Say you clicked on a particularly trick popup add, opening. Therefore, a scenario like this can happen. The only trouble is that the browser automatically includes any relevant cookies stored for a domain when another request is made to that exact domain. Instead, the API will recognize the stored session cookie upon further HTTP requests. And every time you re-visit the tab, and click around the app, you don’t have to sign in again. And this is great! The session cookie gets stored. In this case, your browser would store a relevant session cookie for the. For instance, it’s feasible that you would sign into a web app like. This is especially useful for authentication, and setting sessions. In this maneuver, a malicious website attempts to take advantage of the browser’s cookie storage system.įor every HTTP request to a domain, the browser attaches any HTTP cookies associated with that domain. The same-origin policy fights one of the most common cyber attacks out there: cross-site request forgery. ![]() ![]() The error stems from a security mechanism that browsers implement called the same-origin policy. Why was the CORS error there in the first place?
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