Cross-origin resource sharing

File:Flowchart_showing_Simple_and_Preflight_XHR.svg (XHR) through CORS.]]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.{{cite web |title=Cross-Origin Resource Sharing (CORS) - HTTP {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS |website=developer.mozilla.org |access-date=7 June 2023 |date=10 May 2023}}

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.

1. The browser sends the GET request with an extra Origin HTTP header to service.example.com containing the domain that served the parent page:

   Origin: http://www.example.com

2. The server at service.example.com sends one of these three responses:
3. * The requested data along with an 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

4. * The requested data along with an Access-Control-Allow-Origin (ACAO) header with a wildcard indicating that the requests from all domains are allowed:

   Access-Control-Allow-Origin: *

5. * An error page if the server does not allow a cross-origin request{{Cite web |date=2023-05-10 |title=CORS errors - HTTP {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors |access-date=2023-07-04 |website=developer.mozilla.org |language=en-US}}

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.[https://fetch.spec.whatwg.org/#cors-protocol-and-credentials]. W3.org. Retrieved on 2021-31-07.

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
• Host
• 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:

• Blink- and Chromium-based browsers (Chrome 28+,{{cite web |title=Blink | date=April 2013 |publisher=QuirksBlog |url=http://www.quirksmode.org/blog/archives/2013/04/blink.html |access-date=4 April 2013}}{{cite news |title=Google going its own way, forking WebKit rendering engine |publisher=Ars Technica | date=April 2013 |url= https://arstechnica.com/information-technology/2013/04/google-going-its-own-way-forking-webkit-rendering-engine/ |access-date=4 April 2013}} Opera 15+, Amazon Silk, Android's 4.4+ WebView and Qt's WebEngine)
• Gecko 1.9.1 (Firefox 3.5,{{cite web |url=https://developer.mozilla.org/En/HTTP_access_control |title=HTTP access control (CORS) - MDN |publisher=Developer.mozilla.org |access-date=2012-07-05 |archive-url=https://web.archive.org/web/20100527153021/https://developer.mozilla.org/En/HTTP_access_control |archive-date=2010-05-27 |url-status=dead }} SeaMonkey 2.0{{cite web |url=https://developer.mozilla.org/en/Gecko |title=Gecko - MDN |publisher=Developer.mozilla.org |date=2012-06-08 |access-date=2012-07-05 |archive-date=2012-08-03 |archive-url=https://web.archive.org/web/20120803092112/https://developer.mozilla.org/en/Gecko |url-status=dead }}) and above.
• MSHTML/Trident 6.0 (Internet Explorer 10) has native support.{{cite web|author1=Tony Ross |author2=Program Manager |author3=Internet Explorer |url=http://blogs.msdn.com/b/ie/archive/2012/02/09/cors-for-xhr-in-ie10.aspx |title=CORS for XHR in IE10 |publisher=MSDN |date=2012-02-09 |access-date=2012-12-14}} MSHTML/Trident 4.0 & 5.0 (Internet Explorer 8 & 9) provide partial support via the XDomainRequest object.{{cite web|url=https://hacks.mozilla.org/2009/07/cross-site-xmlhttprequest-with-cors/ |title=cross-site xmlhttprequest with CORS |publisher=MOZILLA |access-date=2012-09-05}}
• Presto-based browsers (Opera) implement CORS as of Opera 12.00{{cite web |author=David Honneffer, Documentation Specialist |url=http://www.opera.com/docs/changelogs/unix/1200/ |title=12.00 for UNIX Changelog |publisher=Opera |date=2012-06-14 |access-date=2012-07-05 |archive-url=https://web.archive.org/web/20120618170555/http://www.opera.com/docs/changelogs/unix/1200/ |archive-date=2012-06-18 |url-status=dead }} and Opera Mobile 12, but not Opera Mini.{{cite web|author=David Honneffer, Documentation Specialist |url=http://www.opera.com/docs/specs/presto2.10/#m210-236 |title=Opera Software: Web specifications support in Opera Presto 2.10 |publisher=Opera.com |date=2012-04-23 |access-date=2012-07-05}}
• WebKit (Initial revision uncertain, Safari 4 and above,{{cite web|author=on July 6, 2009 by Arun Ranganathan |url=https://hacks.mozilla.org/2009/07/cross-site-xmlhttprequest-with-cors/ |title=cross-site xmlhttprequest with CORS ✩ Mozilla Hacks – the Web developer blog |publisher=Hacks.mozilla.org |date=2009-07-06 |access-date=2012-07-05}} Google Chrome 3 and above, possibly earlier).{{cite web |url=http://osvdb.org/59940 |archive-url=https://archive.today/20120719142244/http://osvdb.org/59940 |url-status=dead |archive-date=2012-07-19 |title=59940: Apple Safari WebKit Cross-Origin Resource Sharing Bypass |publisher=Osvdb.org |access-date=2012-07-05 }}
• Microsoft Edge All versions.{{cite web|title=Microsoft Edge deverloper's guide|date=21 December 2023 |url=https://docs.microsoft.com/en-us/microsoft-edge/dev-guide/performance/xmlhttprequest/}}

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{{cite web|url=https://www.w3.org/TR/2004/WD-voicexml21-20040323/#sec-data-security |title=Voice Extensible Markup Language (VoiceXML) 2.1 |publisher=W3.org |date=2004-03-23 |access-date=2012-07-05}} 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.{{cite web|url=http://www.w3.org/TR/2005/NOTE-access-control-20050613/ |title=Authorizing Read Access to XML Content Using the Processing Instruction 1.0 |publisher=W3.org |access-date=2012-07-05}} 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.{{cite web|title=Authorizing Read Access to XML Content Using the Processing Instruction 1.0 W3C - Working Draft 17 May 2006|url=http://www.w3.org/TR/2006/WD-access-control-20060517/|publisher=W3.org|access-date=17 August 2015}} In March 2009 the draft was renamed to "Cross-Origin Resource Sharing"{{cite web|title=Cross-Origin Resource Sharing - W3C Working Draft 17 March 2009|url=http://www.w3.org/TR/2009/WD-cors-20090317/|publisher=W3.org|access-date=17 August 2015}} and in January 2014 it was accepted as a W3C Recommendation.{{cite web|title=Cross-Origin Resource Sharing - W3C Recommendation 16 January 2014|url=http://www.w3.org/TR/2014/REC-cors-20140116/|publisher=W3.org|access-date=17 August 2015}}

CORS can be used as a modern alternative to the JSONP pattern. The benefits of CORS are:

• While JSONP supports only the GET request method, CORS also supports other types of HTTP requests.
• CORS enables a web programmer to use regular XMLHttpRequest, which supports better error handling than JSONP.
• 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.

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.{{cite web|url=http://caniuse.com/#feat=cors |title=When can I use... Cross Origin Resource Sharing |publisher=caniuse.com |access-date=2012-07-12}}

• Content Security Policy
• Cross-document messaging
• Cross site leaks

{{Reflist}}

• [https://fetch.spec.whatwg.org/ Fetch Living Standard] (the current specification for CORS)
• [https://benjaminhorn.io/code/setting-cors-cross-origin-resource-sharing-on-apache-with-correct-response-headers-allowing-everything-through/ Setting CORS on Apache with correct response headers allowing everything through]{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }}
• [https://enable-cors.org/server.html Detailed how-to information for enabling CORS support in various (web) servers]
• [http://www.html5rocks.com/en/tutorials/cors/ HTML5 Rocks explains how CORS works in detail]
• [https://helpertools.app/web-security/view/cors-scanner Online CORS misconfiguration scanner] {{Webarchive|url=https://web.archive.org/web/20200810161738/https://helpertools.app/web-security/view/cors-scanner |date=2020-08-10 }}

{{Web interfaces}}

Category:Ajax (programming)

Category:World Wide Web Consortium standards