The CORS specification requires browsers to preflight requests that: A server can set a value of “*” in this header to indicate that it is a public resource that allows any origin. When the browser sees that the Access-Control-Allow-Origin value matches the domain of the page, it will permit the response to be processed. A CORS-enabled server receiving this request will include these headers in its response: Note that the browser will only include the Origin header when the request is cross-origin. For example to retrieve the resource called some-resource at using the jQuery Ajax API, a developer would simply write :Īnd the browser will issue a request with the following headers: Simple Requestsįor simple cross-site requests (i.e., GETs and POSTs that don’t set custom headers and the request body is plain text or form data), the browser simply includes additional Origin and Referrer headers indicating the requesting domain. The beauty of this mechanism is that it is automatically handled by the browser and web application developers do not need to concern themselves with its details. The server must support CORS and indicate that the domain of the client making the request is permitted to do so. This exchange of headers is what makes CORS a secure mechanism. These headers indicate the origin of the request and the server must indicate via headers in the response whether it will serve resources to this origin. The CORS mechanism works by adding HTTP headers to cross-domain HTTP requests and responses. Historically, for security reasons these types of requests have been prohibited by browsers. If the script on your page is running from domain and would like to request a resource via an XmlHttpRequest or XDomainRequst from domain, this is a cross-origin request. Internet Explorer 10 now has native support. It initially appeared in Firefox 3.5, Safari 4, and Chrome 3. With the emergence of the Cross Origin Resource Sharing (CORS) specification, now a candidate for W3C Recommendation, web application developers have a browser-supported mechanism to make XmlHttpRequests to another domain in a secure manner.Īs of this writing, we can finally say that CORS is supported by all major browsers. Over the years, various techniques have been employed to work around this security restriction, such as server-side proxies, JSONP, and iframe proxies using post message. Up until recently, this had not been possible due to browser-enforced, same-origin security policies for JavaScript. This is especially true if you are part of a large enterprise with distributed sub-domained resources. When building complex client-side applications, at some point it usually becomes necessary to make Ajax requests to domains other than the one from which your page originated.
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