First, it sends a preliminary, so-called “preflight” request, to ask for permission.Ī preflight request uses the method OPTIONS, no body and three headers: So, to avoid misunderstandings, any “unsafe” request – that couldn’t be done in the old times, the browser does not make such requests right away. They can take it into account when checking access rights. So there may still exist webservices that treat a non-standard method as a signal: “That’s not a browser”. Some time ago no one could even imagine that a webpage could make such requests. We can use any HTTP-method: not just GET/POST, but also PATCH, DELETE and others. With such an Access-Control-Expose-Headers header, the script is allowed to read the Content-Encoding and API-Key headers of the response. If a request is cross-origin, the browser always adds the Origin header to it.įor instance, if we request from, the headers will look like:Īccess-Control-Expose-Headers: Content-Encoding,API-Key When we try to make a unsafe request, the browser sends a special “preflight” request that asks the server – does it agree to accept such cross-origin requests, or not?Īnd, unless the server explicitly confirms that with headers, an unsafe request is not sent. So an old server may assume that such requests come from a privileged source, “because a webpage is unable to send them”. For a long time JavaScript was unable to do such requests. So, even a very old server should be ready to accept a safe request.Ĭontrary to that, requests with non-standard headers or e.g. The essential difference is that a safe request can be made with a or a, without any special methods. Content-Type with the value application/x-For instance, a request with PUT method or with an API-Key HTTP-header does not fit the limitations.Safe headers – the only allowed custom headers are:.Safe Requests are simpler to make, so let’s start with them.Ī request is safe if it satisfies two conditions: There are two types of cross-origin requests: But as a result of long discussions, cross-origin requests were allowed, but with any new capabilities requiring an explicit allowance by the server, expressed in special headers. There are still services that provide such access, as it works even for very old browsers.Īfter a while, networking methods appeared in browser JavaScript.Īt first, cross-origin requests were forbidden. And, when both sides agree, it’s definitely not a hack. That works, and doesn’t violate security, because both sides agreed to pass the data this way. When the remote script loads and executes, gotWeather runs, and, as it’s our function, we have the data. The expected answer from the server looks like this: Let’s say we, at our site, need to get the data from, such as the weather:įirst, in advance, we declare a global function to accept the data, e.g. intended to expose data for this kind of access, then a so-called “JSONP (JSON with padding)” protocol was used. It’s possible to execute a script from any website. A script could have any src, with any domain, like. Using scriptsĪnother trick was to use a script tag. Right now there’s no point to go into details, let these dinosaurs rest in peace.

So the communication with the iframe was technically possible. To be precise, there were actually tricks for that, they required special scripts at both the iframe and the page. But as it’s forbidden to access the content of an from another site, it wasn’t possible to read the response.

So, it was possible to make a GET/POST request to another site, even without networking methods, as forms can send data anywhere. People submitted it into, just to stay on the current page, like this: One way to communicate with another server was to submit a there.

A variety of tricks were invented to work around the limitation and make requests to other websites. It was a toy language to decorate a web page.īut web developers demanded more power.

JavaScript also did not have any special methods to perform network requests at that time. an evil script from website could not access the user’s mailbox at website. That simple, yet powerful rule was a foundation of the internet security. Let’s make a very brief historical digression.įor many years a script from one site could not access the content of another site. Why is CORS needed? A brief historyĬORS exists to protect the internet from evil hackers. That policy is called “CORS”: Cross-Origin Resource Sharing. The core concept here is origin – a domain/port/protocol triplet.Ĭross-origin requests – those sent to another domain (even a subdomain) or protocol or port – require special headers from the remote side.

Access control allow origin fetch