Cross-origin resource sharing
Cross-origin resource sharing (CORS) is a mechanism that allows restricted resources on a web page to be requested from another domain outside the domain from which the first resource was served. A web page may freely embed cross-origin images, stylesheets, scripts, iframes, and videos. Certain "cross-domain" requests, notably Ajax requests, are forbidden by default by the same-origin security policy.
CORS defines a way in which a browser and server can interact to determine whether or not it is safe to allow the cross-origin request. It allows for more freedom and functionality than purely same-origin requests, but is more secure than simply allowing all cross-origin requests. The specification for CORS was originally published as a W3C Recommendation but that document is obsolete. The current actively-maintained specification that defines CORS is WHATWG's Fetch Living Standard.
How CORS works
The CORS standard describes new HTTP headers which provide browsers a way to request remote URLs only when they have permission. Although some validation and authorization can be performed by the server, it is generally the browser's responsibility to support these headers and honor the restrictions they impose.
For Ajax and HTTP request methods that can modify data (usually HTTP methods other than GET, or for POST usage with certain MIME types), the specification mandates that browsers "preflight" the request, soliciting supported methods from the server with an HTTP OPTIONS request method, and then, upon "approval" from the server, sending the actual request with the actual HTTP request method. Servers can also notify clients whether "credentials" (including Cookies and HTTP Authentication data) should be sent with requests.
Suppose a user visits http://www.example.com and the page attempts a cross-origin request to fetch the user's data from http://service.example.com. A CORS-compatible browser will attempt to make a cross-origin request to service.example.com as follows.
- The browser sends the OPTIONS request with an
OriginHTTP header to service.example.com containing the domain that served the parent page:
- The server at service.example.com may respond with:
Access-Control-Allow-Origin(ACAO) header in its response indicating which origin sites are allowed. For example:
Access-Control-Allow-Origin: http://www.example.comSince www.example.com matches the parent page, the browser then performs the cross-origin request.
Access-Control-Allow-Origin(ACAO) header with a wildcard that allows all domains:
- An error page if the server does not allow a cross-origin request
A wildcard same-origin policy is appropriate when a page or API response is considered completely public content and it is intended to be accessible to everyone, including any code on any site. For example, a freely-available web font on a public hosting service like Google Fonts.
A wildcard same-origin policy is also widely and appropriately used in the object-capability model, where pages have unguessable URLs and are meant to be accessible to anyone who knows the secret.
The value of "*" is special in that it does not allow requests to supply credentials, meaning it does not allow HTTP authentication, client-side SSL certificates, or cookies to be sent in the cross-domain request.
Note that in the CORS architecture, the ACAO header is being set by the external web service (service.example.com), not the original web application server (www.example.com). Here, service.example.com uses CORS to permit the browser to authorize www.example.com to make requests to service.example.com.
When performing certain types of cross-domain Ajax requests, modern browsers that support CORS will insert an extra "preflight" request to determine whether they have permission to perform the action.
OPTIONS / Host: service.example.com Origin: http://www.example.com
If service.example.com is willing to accept the action, it may respond with the following headers:
Access-Control-Allow-Origin: http://www.example.com Access-Control-Allow-Methods: PUT, DELETE
The HTTP headers that relate to CORS are
CORS is supported by all browsers based on the following layout engines:
- Blink- and Chromium-based browsers (Chrome 28+, Opera 15+, Amazon Silk, Android's 4.4+ WebView and Qt's WebEngine)
- Gecko 1.9.1 (Firefox 3.5, SeaMonkey 2.0) and above.
- MSHTML/Trident 6.0 (Internet Explorer 10) has native support. MSHTML/Trident 4.0 & 5.0 (Internet Explorer 8 & 9) provide partial support via the XDomainRequest object.
- Presto-based browsers (Opera) implement CORS as of Opera 12.00 and Opera Mobile 12, but not Opera Mini.
- WebKit (Initial revision uncertain, Safari 4 and above, Google Chrome 3 and above, possibly earlier).
- Microsoft Edge All versions. 
Cross-origin support was originally proposed by Matt Oshry, Brad Porter, and Michael Bodell of Tellme Networks in March 2004 for inclusion in VoiceXML 2.1 to allow safe cross-origin data requests by VoiceXML browsers. The mechanism was deemed general in nature and not specific to VoiceXML and was subsequently separated into an implementation NOTE. The WebApps Working Group of the W3C with participation from the major browser vendors began to formalize the NOTE into a W3C Working Draft on track toward formal W3C Recommendation status.
CORS vs JSONP
CORS can be used as a modern alternative to the JSONP pattern. While JSONP supports only the GET request method, CORS also supports other types of HTTP requests. Using CORS enables a web programmer to use regular XMLHttpRequest, which supports better error handling than JSONP. On the other hand, JSONP works on legacy browsers which predate CORS support. CORS is supported by most modern web browsers. Also, while JSONP can cause cross-site scripting (XSS) issues when the external site is compromised, CORS allows websites to manually parse responses to increase security.
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- MDN HTTP access control (CORS) article
- Setting CORS on Apache with correct response headers allowing everything through
- Detailed how-to information for enabling CORS support in various (web) servers
- HTML5 Rocks explains how CORS works in detail
- W3C CORS for Developers guide