Cross-origin resource sharing (CORS) is a mechanism to safely bypass the same-origin policy, that is, it allows a web page to access restricted resources from a server on a domain different than the domain that served the web page.
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 it is safe to allow the cross-origin request. [1] 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 is included as part of the WHATWG's Fetch Living Standard. [2] This specification describes how CORS is currently implemented in browsers. [3] An earlier specification was published as a W3C Recommendation. [4]
For HTTP requests made from JavaScript that can't be made by using a <form> tag pointing to another domain or containing non-safelisted headers, 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. [5]
Suppose a user visits http://www.example.com and the page attempts a cross-origin request to fetch data from http://service.example.com. A CORS-compatible browser will attempt to make a cross-origin request to service.example.com as follows.
Origin
HTTP header to service.example.com containing the domain that served the parent page:Origin: http://www.example.com
Access-Control-Allow-Origin
(ACAO) header in its response indicating the requests from the origin are allowed. For example in this case it should be: Access-Control-Allow-Origin: http://www.example.com
Access-Control-Allow-Origin
(ACAO) header with a wildcard indicating that the requests from all domains are allowed: Access-Control-Allow-Origin: *
A wildcard same-origin policy is appropriate when a page or API response is intended to be accessible to any code on any site. A freely available web font on a public hosting service like Google Fonts is an example.
The value of "*" is special in that it does not allow requests to supply credentials, meaning that it does not allow HTTP authentication, client-side SSL certificates, or cookies to be sent in the cross-domain request. [7]
Note that in the CORS architecture, the Access-Control-Allow-Origin 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.
If a site specifies the header "Access-Control-Allow-Credentials:true", third-party sites may be able to carry out privileged actions and retrieve sensitive information.
When performing certain types of cross-domain Ajax requests, modern browsers that support CORS will initiate an extra "preflight" request to determine whether they have permission to perform the action. Cross-origin requests are preflighted this way because they may have implications to user data.
OPTIONS / Host: service.example.com Origin: http://www.example.com Access-Control-Request-Method: PUT
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
The browser will then make the actual request. If service.example.com does not accept cross-site requests from this origin then it will respond with error to the OPTIONS request and the browser will not make the actual request.
The HTTP headers that relate to CORS are:
Origin
Access-Control-Request-Method
Access-Control-Request-Headers
Access-Control-Allow-Origin
Access-Control-Allow-Credentials
Access-Control-Expose-Headers
Access-Control-Max-Age
Access-Control-Allow-Methods
Access-Control-Allow-Headers
CORS is supported by all browsers based on the following layout engines:
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 [19] 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. [20] 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.
In May 2006 the first W3C Working Draft was submitted. [21] In March 2009 the draft was renamed to "Cross-Origin Resource Sharing" [22] and in January 2014 it was accepted as a W3C Recommendation. [23]
CORS can be used as a modern alternative to the JSONP pattern. The benefits of CORS are:
GET
request method, CORS also supports other types of HTTP requests.The main advantage of JSONP was its ability to work on legacy browsers which predate CORS support (Opera Mini and Internet Explorer 9 and earlier). CORS is now supported by most modern web browsers. [24]
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comprises the name of the header itself, the header value that a given user agent uses to identify itself is colloquially known as its user agent string. The user agent for the operator of a computer used to access the Web has encoded within the rules that govern its behavior the knowledge of how to negotiate its half of a request-response transaction; the user agent thus plays the role of the client in a client–server system. Often considered useful in networks is the ability to identify and distinguish the software facilitating a network session. For this reason, the User-Agent HTTP header exists to identify the client software to the responding server.
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, where <credentials>
is the Base64 encoding of ID and password joined by a single colon :
.
XMLHttpRequest (XHR) is an API in the form of a JavaScript object whose methods transmit HTTP requests from a web browser to a web server. The methods allow a browser-based application to send requests to the server after page loading is complete, and receive information back. XMLHttpRequest is a component of Ajax programming. Prior to Ajax, hyperlinks and form submissions were the primary mechanisms for interacting with the server, often replacing the current page with another one.
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element, which is an element intended to load ordinary JavaScript. It was proposed by Bob Ippolito in 2005. JSONP enables sharing of data bypassing same-origin policy, which disallows running JavaScript code to read media DOM elements or XMLHttpRequest data fetched from outside the page's originating site. The originating site is indicated by a combination of URI scheme, hostname, and port number.
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HTML audio is a subject of the HTML specification, incorporating audio |speech to text]], all in the browser.
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