I'm diving deeper into developing RESTful APIs and have so far worked with a few different frameworks to achieve this. Of course I've run into the same-origin policy, and now I'm wondering how web servers (rather than web browsers) enforce it. From what I understand, some enforcing seems to happen in the browser's end (e.g., honoring an Access-Control-Allow-Origin header received from a server). But what about the server?

For example, let's say a web server is hosting a Javascript web app that accesses an API, also hosted on that server. I assume that the server would enforce the same-origin policy --- so that only the javascript that is hosted on that server would be allowed to access the API. This would prevent someone else from writing a javascript client for that API and hosting it on another site, right? So how would a web server be able to stop a malicious client that would try to make AJAX requests to its API endpoints while claiming to be running javascript that originated from that same web server? What's the way most popular servers (Apache, nginx) protect against this kind of attack? Or is my understanding of this somehow off the mark?

Or is the cross-origin policy only enforced on the client end?

The same origin policy is a wholly client-based restriction, and is primarily engineered to protect users, not services. All or most browsers include a command-line switch or configuration option to to turn it off. The SOP is like seat belts in a car: they protect the rider in the car, but anyone can freely choose not to use them. Certainly don't expect a person's seat belt to stop them from getting out of their car and attacking you (or accessing your Web service).

Suppose I write a program that accesses your Web service. It's just a program that sends TCP messages that include HTTP requests. You're asking for a server-side mechanism to distinguish between requests made by my program (which can send anything) and requests made by a browser that has a page loaded from a permitted origin. It simply can't be done; my program can always send a request identical to one formed by a Web page.

The same-origin policy was invented because it prevents code from one website from accessing credential-restricted content on another site. Ajax requests are by default sent with any auth cookies granted by the target site. For example, suppose I accidentally load http://evil.com/, which sends a request for http://mail.google.com/. If the SOP were not in place, and I was signed into Gmail, the script at evil.com could see my inbox. If the site at evil.com wants to load mail.google.com without my cookies, it can just use a proxy server; the public contents of mail.google.com are not a secret (but the contents of mail.google.com when accessed with my cookies are a secret).
