cross-origin xhr and same-origin-policy

Question

This is a concept that I thought I understood, but recently found out I had all wrong. I've looked all around the internet and find plenty of examples of small details and code snippets, but I still lack an understanding of what it prevents and why it prevents it and for whose sake. So this is more of a request for a high-level explanation than a question.

Anyways, here's what I THINK I understand about it:

Let's say I have domain A.com and domain B.com. Each on their own apache servers with their own IP addresses. I load an html file from domain A.com into a browser. The browser executes a POST XMLHttpRequest to B.com/doStuff.php, which fails because the same-domain-policy was set.

So: Who's same-domain-policy is relevant? I think the answer is B.com/doStuff.php's... right? So when A sends the request, B checks the request headers for an origin and says "whoops, different domain, won't listen to you". Or does A send the request, B responds with headers that specify "same-domain-policy", and then the browser checks that because same-domain-policy was specified, and the domain from the headers of the A request don't match the one from the B request the BROWSER refuses to send out the xhr?

If that's the case, it seems that the point of not allowing cross-origin-requests is because "I don't want anyone other than me accessing my API". Is that all? Because wouldn't you want to solve that with some kind of authentication instead? Couldn't someone just construct an HTTP request with a fake origin header (simply lie)?

Or is this somehow supposed to protect the user? If that's the case, how is preventing them from calling your API ever going to protect anyone?

I'm so confused...

Answer 1

Who's same-domain-policy is relevant?

The server receiving the request decides.

... the BROWSER refuses to send out the xhr?

No, the server refuses to respond. To be more exact, in modern browsers it is done by preflighted requests. It means that for each cross-origin request, first an OPTIONS request is sent automatically by the browser whose headers are the exact same as the intended request will have but with no request body. The server responds also with headers only. Access-Control headers in the response will let the client browser know whether the request would be fulfilled according to the server's policies. In a sense the browser is preventing the request, but only because it already exchanged a request/response pair with the server and knows that there would be no point to attempt the request. If you forge a request in such a case, the server will still deny serving it.

Answer 2

The idea is that you don't want to access server b from server A via Javascript. If you're interacting with an API, you would use javascript to make a call to your own server's backend code, which would then make a call to the other server.