Cross-Origin Resource Sharing (CORS)

Cross-Origin Resource Sharing (CORS) — механизм, использующий дополнительные HTTP-заголовки, чтобы дать возможность агенту пользователя получать разрешения на доступ к выбранным ресурсам с сервера на источнике (домене), отличном от того, что сайт использует в данный момент. Говорят, что агент пользователя делает запрос с другого источника (cross-origin HTTP request), если источник текущего документа отличается от запрашиваемого ресурса доменом, протоколом или портом.

Пример cross-origin запроса: HTML страница, обслуживаемая сервером с http://domain-a.com, запрашивает <img> src по адресу http://domain-b.com/image.jpg. Сегодня многие страницы загружают ресурсы вроде CSS-стилей, изображений и скриптов с разных доменов, соответствующих разным сетям доставки контента (Content delivery networks, CDNs).

В целях безопасности браузеры ограничивают cross-origin запросы, инициируемые скриптами. Например, XMLHttpRequest и Fetch API следуют политике одного источника (same-origin policy). Это значит, что web-приложения, использующие такие API, могут запрашивать HTTP-ресурсы только с того домена, с которого были загружены, пока не будут использованы CORS-заголовки.

Механизм CORS поддерживает кросс-доменные запросы и передачу данных между браузером и web-серверами по защищенному соединению. Современные браузеры используют CORS в API-контейнерах, таких как XMLHttpRequest или Fetch, чтобы снизить риски, присущие запросам с других источников.

Кто должен читать данную статью?

На самом деле, все.

Конкретнее, эта статья для web-администраторов, разработчиков серверной стороны и front-end разработчиков. Современные браузеры поддерживают клиентские компоненты cross-origin обмена, включая заголовки и соблюдение правил полотики. Но этот новый стандарт означает, что сервера так же должны поддерживать новые заголовки запросов и ответов. Другая статья для разработчиков серверной части, описывающая перспективы сross-origin обмена на стороне сервара (с примерами кода на PHP), к дополнительному прочтению.

Какие запросы используют CORS?

Этот cтандарт сross-origin обмена используется для разрешения кросс-сайтовых HTTP запросов для:

• Вызова XMLHttpRequest или Fetch APIs в кросс-сайт манере, как описано выше.
• Web Fonts (для кросс-доменного использования шрифтов в @font-face в рамках CSS), чтобы серверы могли разворачивать TrueType шрифты, которые могут быть загружены только кросс-сайт и использованы web-сайтами, которым это разрешено.
• WebGL текстуры.
• Фреймы с изображениями/видео, добавленными в канвас с помощью drawImage.
• Стили (для CSSOM доступа).
• Скрипты (для  отключенных исключений).

Эта статья описывает общие понятия Cross-Origin Resource Sharing и включает обсуждение необходимых HTTP заголовков.

Обзор функциональности

Стандарт Cross-Origin Resource Sharing работает с помощью добавления новых HTTP-заголовков, которые позволяют серверам описывать набор источников, которым разрешено читать информацию, запрашиваемую web-браузером. В частности, для методов HTTP-запросов, которые могут привести к сторонним эффектам над данными сервера (в частности, для HTTP методов, отличных от GET или для POST запросов, использующих определнные MIME-типы), спецификация требует, чтобы браузеры "предпроверяли" запрос, запрашивая поддерживающие методы с сервера с помощью метода HTTP-запроса OPTIONS и затем, поверх "подтверждения" с сервера, отсылали фактический запрос с фактическим методом HTTP-запроса. Сервера так же могут оповещать клиентов должны ли "полномочия" (включая Cookies и HTTP Authentication данные) быть отправлены с запросом.

Следующая секция описывает сценарии, а так же предоставляет анализ использования HTTP-заголовков. 

Примеры сценариев управления доступом

Здесь мы рассмотрим три сценария, которые иллюстрируют как Cross-Origin Resource Sharing работает. Каждый сценарий использует объект XMLHttpRequest, который может быть использован для межсайтового взаимодействия, в любом, поддерживающем данный объект, браузере.

Фрагменты JavaScript кода, включенные в эти секции (а также фрагменты кода, отвечающие за корректную обработку межсерверных запросов, которые запускаются на сервере) могут быть испытаны "в действии" на http://arunranga.com/examples/access-control/, и будут работать в браузерах, которые поддерживают XMLHttpRequest.

Обсуждение Cross-Origin Resource Sharing с точки зрения сервера (включая фрагменты кода на PHP) может быть найдено в статье Server-Side Access Control (CORS).

Простые запросы

Некоторые запросы не заставляют срабатывать CORS preflight. Они называются “простыми запросами” в данной статье, хотя Fetch спецификация, определяющая CORS, не использует этот термин. Запрос, для которого не срабатывает CORS preflight— так называемый “простой запросы”—это запрос, удовлетворяющий следующим условиям:

• Допустимые методы для запроса:
  • GET
  • HEAD
  • POST
• Кроме заголовков, которые автоматические проставляются user-agent'ом (например, Connection, User-Agent, или любой другой заголовок с именем, определенным в спецификации метода Fetch в секции “Запрещенные имена заголовков (которые нельзя изменить программно)”), допустимыми заголовками, которые могут быть проставлены вручную, являются те заголовки, которые определены спецификацией метода Fetch как “CORS-безопасные заголовки запроса”, такие как:
  • Accept
  • Accept-Language
  • Content-Language
  • Content-Type (но учитывайте примечание ниже)
  • Last-Event-ID
  • DPR
  • Save-Data
  • Viewport-Width
  • Width
• Допустимыми значениями заголовка Content-Type являются:
  • application/x-www-form-urlencoded
  • multipart/form-data
  • text/plain
• Не должно быть зарегистрированы обработчики событий на любой объект XMLHttpRequestUpload используемый в запросе; это достигается использованием свойства XMLHttpRequest.upload.
• В запросе не должен использоваться объект типа ReadableStream.

For example, suppose web content on domain http://foo.example wishes to invoke content on domain http://bar.other. Code of this sort might be used within JavaScript deployed on foo.example:

var invocation = new XMLHttpRequest();
var url = 'http://bar.other/resources/public-data/';
   
function callOtherDomain() {
  if(invocation) {    
    invocation.open('GET', url, true);
    invocation.onreadystatechange = handler;
    invocation.send(); 
  }
}

This will lead to a simple exchange between the client and the server, using CORS headers to handle the privileges:

Let us look at what the browser will send to the server in this case, and let's see how the server responds:

GET /resources/public-data/ HTTP/1.1
Host: bar.other
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20081130 Minefield/3.1b3pre
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
Referer: http://foo.example/examples/access-control/simpleXSInvocation.html
Origin: http://foo.example


HTTP/1.1 200 OK
Date: Mon, 01 Dec 2008 00:23:53 GMT
Server: Apache/2.0.61 
Access-Control-Allow-Origin: *
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: application/xml

[XML Data]

Lines 1 - 10 are headers sent. The main HTTP request header of note here is the Origin header on line 10 above, which shows that the invocation is coming from content on the domain http://foo.example.

Lines 13 - 22 show the HTTP response from the server on domain http://bar.other. In response, the server sends back an Access-Control-Allow-Origin header, shown above in line 16. The use of the Origin header and of Access-Control-Allow-Origin show the access control protocol in its simplest use. In this case, the server responds with a Access-Control-Allow-Origin: * which means that the resource can be accessed by any domain in a cross-site manner. If the resource owners at http://bar.other wished to restrict access to the resource to requests only from http://foo.example, they would send back:

Access-Control-Allow-Origin: http://foo.example

Note that now, no domain other than http://foo.example (identified by the ORIGIN: header in the request, as in line 10 above) can access the resource  in a cross-site manner.  The Access-Control-Allow-Origin header should contain the value that was sent in the request's Origin header. 

Preflighted requests

Unlike “simple requests” (discussed above), "preflighted" requests first send an HTTP request by the OPTIONS method to the resource on the other domain, in order to determine whether the actual request is safe to send. Cross-site requests are preflighted like this since they may have implications to user data.

In particular, a request is preflighted if any of the following conditions is true:

• If the request uses any of the following methods:
  • PUT
  • DELETE
  • CONNECT
  • OPTIONS
  • TRACE
  • PATCH
• Or if, apart from the headers set automatically by the user agent (for example, Connection, User-Agent, or any of the other header with a name defined in the Fetch spec as a “forbidden header name”), the request includes any headers other than those which the Fetch spec defines as being a “CORS-safelisted request-header”, which are the following:
  • Accept
  • Accept-Language
  • Content-Language
  • Content-Type (but note the additional requirements below)
  • Last-Event-ID
  • DPR
  • Save-Data
  • Viewport-Width
  • Width
• Or if the Content-Type header has a value other than the following:
  • application/x-www-form-urlencoded
  • multipart/form-data
  • text/plain
• Or if one or more event listeners are registered on an XMLHttpRequestUpload object used in the request.
• Or if a ReadableStream object is used in the request.

The following is an example of a request that will be preflighted.

var invocation = new XMLHttpRequest();
var url = 'http://bar.other/resources/post-here/';
var body = '<?xml version="1.0"?><person><name>Arun</name></person>';
    
function callOtherDomain(){
  if(invocation)
    {
      invocation.open('POST', url, true);
      invocation.setRequestHeader('X-PINGOTHER', 'pingpong');
      invocation.setRequestHeader('Content-Type', 'application/xml');
      invocation.onreadystatechange = handler;
      invocation.send(body); 
    }
}

......

In the example above, line 3 creates an XML body to send with the POST request in line 8.  Also, on line 9, a "customized" (non-standard) HTTP request header is set (X-PINGOTHER: pingpong).  Such headers are not part of the HTTP/1.1 protocol, but are generally useful to web applications.  Since the request uses a Content-Type of application/xml, and since a custom header is set, this request is preflighted.

(Note: as described below, the actual POST request does not include the Access-Control-Request-* headers; they are needed only for the OPTIONS request.)

Let's take a look at the full exchange between client and server. The first exchange is the preflight request/response:

OPTIONS /resources/post-here/ HTTP/1.1
Host: bar.other
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20081130 Minefield/3.1b3pre
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
Origin: http://foo.example
Access-Control-Request-Method: POST
Access-Control-Request-Headers: X-PINGOTHER, Content-Type


HTTP/1.1 200 OK
Date: Mon, 01 Dec 2008 01:15:39 GMT
Server: Apache/2.0.61 (Unix)
Access-Control-Allow-Origin: http://foo.example
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: X-PINGOTHER, Content-Type
Access-Control-Max-Age: 86400
Vary: Accept-Encoding, Origin
Content-Encoding: gzip
Content-Length: 0
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive
Content-Type: text/plain

Once the preflight request is complete, the real request is sent:

POST /resources/post-here/ HTTP/1.1
Host: bar.other
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20081130 Minefield/3.1b3pre
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
X-PINGOTHER: pingpong
Content-Type: text/xml; charset=UTF-8
Referer: http://foo.example/examples/preflightInvocation.html
Content-Length: 55
Origin: http://foo.example
Pragma: no-cache
Cache-Control: no-cache

<?xml version="1.0"?><person><name>Arun</name></person>


HTTP/1.1 200 OK
Date: Mon, 01 Dec 2008 01:15:40 GMT
Server: Apache/2.0.61 (Unix)
Access-Control-Allow-Origin: http://foo.example
Vary: Accept-Encoding, Origin
Content-Encoding: gzip
Content-Length: 235
Keep-Alive: timeout=2, max=99
Connection: Keep-Alive
Content-Type: text/plain

[Some GZIP'd payload]

Lines 1 - 12 above represent the preflight request with the OPTIONS method. The browser determines that it needs to send this based on the request parameters that the JavaScript code snippet above was using, so that the server can respond whether it is acceptable to send the request with the actual request parameters. OPTIONS is an HTTP/1.1 method that is used to determine further information from servers, and is a safe method, meaning that it can't be used to change the resource. Note that along with the OPTIONS request, two other request headers are sent (lines 10 and 11 respectively):

Access-Control-Request-Method: POST
Access-Control-Request-Headers: X-PINGOTHER, Content-Type

The Access-Control-Request-Method header notifies the server as part of a preflight request that when the actual request is sent, it will be sent with a POST request method. The Access-Control-Request-Headers header notifies the server that when the actual request is sent, it will be sent with a X-PINGOTHER and Content-Type custom headers. The server now has an opportunity to determine whether it wishes to accept a request under these circumstances.

Lines 14 - 26 above are the response that the server sends back indicating that the request method (POST) and request headers (X-PINGOTHER) are acceptable. In particular, let's look at lines 17-20:

Access-Control-Allow-Origin: http://foo.example
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: X-PINGOTHER, Content-Type
Access-Control-Max-Age: 86400

The server responds with Access-Control-Allow-Methods and says that POST, GET, and OPTIONS are viable methods to query the resource in question. Note that this header is similar to the Allow response header, but used strictly within the context of access control.

The server also sends Access-Control-Allow-Headers with a value of "X-PINGOTHER, Content-Type", confirming that these are permitted headers to be used with the actual request. Like Access-Control-Allow-Methods, Access-Control-Allow-Headers is a comma separated list of acceptable headers.

Finally, Access-Control-Max-Age gives the value in seconds for how long the response to the preflight request can be cached for without sending another preflight request. In this case, 86400 seconds is 24 hours. Note that each browser has a maximum internal value that takes precedence when the Access-Control-Max-Age is greater.

Preflighted requests and redirects

Most browsers currently don’t support following redirects for preflighted requests. If a redirect occurs for a preflighted request, most current browsers will report an error message such as the following.

The request was redirected to 'https://example.com/foo', which is disallowed for cross-origin requests that require preflight

Request requires preflight, which is disallowed to follow cross-origin redirect

The CORS protocol originally required that behavior but was subsquently changed to no longer require it. However, most browsers have not yet implemented the change and still exhibit the behavior that was originally required.

So until browsers catch up with the spec, you may be able to work around this limitation by doing one or both of the following:

• change the server-side behavior to avoid the preflight and/or to avoid the redirect—if you have control over the server the request is being made to
• change the request such that it is a simple request that doesn’t cause a preflight

But if it’s not possible to make those changes, then another way that may be possible is to this:

1. Make a simple request to determine (using Response.url for the Fetch API, or XHR.responseURL to determine what URL the real preflighted request would end up at).
2. Make another request (the “real” request) using the URL you obtained from Response.url or XMLHttpRequest.responseURL in the first step.

However, if the request is one that triggers a preflight due to the presence of the `Authorization` header in the request, you won’t be able to work around the limitation using the steps above. And you won’t be able to work around it at all unless you have control over the server the request is being made to.

Requests with credentials

The most interesting capability exposed by both XMLHttpRequest or Fetch and CORS is the ability to make "credentialed" requests that are aware of HTTP cookies and HTTP Authentication information. By default, in cross-site XMLHttpRequest or Fetch invocations, browsers will not send credentials. A specific flag has to be set on the XMLHttpRequest object or the Request constructor when it is invoked.

In this example, content originally loaded from http://foo.example makes a simple GET request to a resource on http://bar.other which sets Cookies. Content on foo.example might contain JavaScript like this:

var invocation = new XMLHttpRequest();
var url = 'http://bar.other/resources/credentialed-content/';
    
function callOtherDomain(){
  if(invocation) {
    invocation.open('GET', url, true);
    invocation.withCredentials = true;
    invocation.onreadystatechange = handler;
    invocation.send(); 
  }
}

Line 7 shows the flag on XMLHttpRequest that has to be set in order to make the invocation with Cookies, namely the withCredentials boolean value. By default, the invocation is made without Cookies. Since this is a simple GET request, it is not preflighted, but the browser will reject any response that does not have the Access-Control-Allow-Credentials: true header, and not make the response available to the invoking web content.

Here is a sample exchange between client and server:

GET /resources/access-control-with-credentials/ HTTP/1.1
Host: bar.other
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20081130 Minefield/3.1b3pre
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
Referer: http://foo.example/examples/credential.html
Origin: http://foo.example
Cookie: pageAccess=2


HTTP/1.1 200 OK
Date: Mon, 01 Dec 2008 01:34:52 GMT
Server: Apache/2.0.61 (Unix) PHP/4.4.7 mod_ssl/2.0.61 OpenSSL/0.9.7e mod_fastcgi/2.4.2 DAV/2 SVN/1.4.2
X-Powered-By: PHP/5.2.6
Access-Control-Allow-Origin: http://foo.example
Access-Control-Allow-Credentials: true
Cache-Control: no-cache
Pragma: no-cache
Set-Cookie: pageAccess=3; expires=Wed, 31-Dec-2008 01:34:53 GMT
Vary: Accept-Encoding, Origin
Content-Encoding: gzip
Content-Length: 106
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive
Content-Type: text/plain


[text/plain payload]

Although line 11 contains the Cookie destined for the content on http://bar.other, if bar.other did not respond with an Access-Control-Allow-Credentials: true (line 19) the response would be ignored and not made available to web content.

Credentialed requests and wildcards

When responding to a credentialed request, the server must specify an origin in the value of the Access-Control-Allow-Origin header, instead of specifying the "*" wildcard.

Because the request headers in the above example include a Cookie header, the request would fail if the value of the Access-Control-Allow-Origin header were "*". But it does not fail: Because the value of the Access-Control-Allow-Origin header is "http://foo.example" (an actual origin) rather than the "*" wildcard, the credential-cognizant content is returned to the invoking web content.

Note that the Set-Cookie response header in the example above also sets a further cookie. In case of failure, an exception—depending on the API used—is raised.

The HTTP response headers

This section lists the HTTP response headers that servers send back for access control requests as defined by the Cross-Origin Resource Sharing specification. The previous section gives an overview of these in action.

Access-Control-Allow-Origin

A returned resource may have one Access-Control-Allow-Origin header, with the following syntax:

Access-Control-Allow-Origin: <origin> | *

The origin parameter specifies a URI that may access the resource. The browser must enforce this. For requests without credentials, the server may specify "*" as a wildcard, thereby allowing any origin to access the resource.

For example, to allow http://mozilla.org to access the resource, you can specify:

Access-Control-Allow-Origin: http://mozilla.org

If the server specifies an origin host rather than "*", then it could also include Origin in the Vary response header to indicate to clients that server responses will differ based on the value of the Origin request header.

Access-Control-Expose-Headers

The Access-Control-Expose-Headers header lets a server whitelist headers that browsers are allowed to access. For example:

Access-Control-Expose-Headers: X-My-Custom-Header, X-Another-Custom-Header

This allows the X-My-Custom-Header and X-Another-Custom-Header headers to be exposed to the browser.

Access-Control-Max-Age

The  Access-Control-Max-Age header indicates how long the results of a preflight request can be cached. For an example of a preflight request, see the above examples.

Access-Control-Max-Age: <delta-seconds>

The delta-seconds parameter indicates the number of seconds the results can be cached.

Access-Control-Allow-Credentials

The Access-Control-Allow-Credentials header Indicates whether or not the response to the request can be exposed when the credentials flag is true.  When used as part of a response to a preflight request, this indicates whether or not the actual request can be made using credentials. Note that simple GET requests are not preflighted, and so if a request is made for a resource with credentials, if this header is not returned with the resource, the response is ignored by the browser and not returned to web content.

Access-Control-Allow-Credentials: true

Credentialed requests are discussed above.

Access-Control-Allow-Methods

The Access-Control-Allow-Methods header specifies the method or methods allowed when accessing the resource. This is used in response to a preflight request. The conditions under which a request is preflighted are discussed above.

Access-Control-Allow-Methods: <method>[, <method>]*

An example of a preflight request is given above, including an example which sends this header to the browser.

Access-Control-Allow-Headers

The Access-Control-Allow-Headers header is used in response to a preflight request to indicate which HTTP headers can be used when making the actual request.

Access-Control-Allow-Headers: <field-name>[, <field-name>]*

The HTTP request headers

This section lists headers that clients may use when issuing HTTP requests in order to make use of the cross-origin sharing feature. Note that these headers are set for you when making invocations to servers. Developers using cross-site XMLHttpRequest capability do not have to set any cross-origin sharing request headers programmatically.

Origin

The Origin header indicates the origin of the cross-site access request or preflight request.

Origin: <origin>

The origin is a URI indicating the server from which the request initiated.  It does not include any path information, but only the server name.

Note that in any access control request, the Origin header is always sent.

Access-Control-Request-Method

The Access-Control-Request-Method is used when issuing a preflight request to let the server know what HTTP method will be used when the actual request is made.

Access-Control-Request-Method: <method>

Examples of this usage can be found above.

Access-Control-Request-Headers

The Access-Control-Request-Headers header is used when issuing a preflight request to let the server know what HTTP headers will be used when the actual request is made.

Access-Control-Request-Headers: <field-name>[, <field-name>]*

Examples of this usage can be found above.

Specifications

Browser compatibility

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Compatibility notes

• Internet Explorer 8 and 9 expose CORS via the XDomainRequest object, but have a full implementation in IE 10. 
• While Firefox 3.5 introduced support for cross-site XMLHttpRequests and Web Fonts, certain requests were limited until later versions. Specifically, Firefox 7 introduced the ability for cross-site HTTP requests for WebGL Textures, and Firefox 9 added support for Images drawn on a canvas using drawImage.

See also

• Code Samples Showing XMLHttpRequest and Cross-Origin Resource Sharing
• Cross-Origin Resource Sharing From a Server-Side Perspective (PHP, etc.)
• Cross-Origin Resource Sharing specification
• XMLHttpRequest
• Fetch API
• Using CORS with All (Modern) Browsers
• Using CORS - HTML5 Rocks
• Stack Overflow answer with “how to” info for dealing with common problems:
  • How to avoid the CORS preflight
  • How to use a CORS proxy to get around “No Access-Control-Allow-Origin header”
  • How to fix “Access-Control-Allow-Origin header must not be the wildcard”