HTTP access control (CORS)

A resource makes a cross-origin HTTP request when it requests a resource from a different domain than the one which served itself. For example, an HTML page served from http://domain-a.com makes an 

<img> src

 request for http://domain-b.com/image.jpg. Many pages on the web today load resources like CSS stylesheets, images and scripts from separate domains.

For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts.  For example, XMLHttpRequest follows the same-origin policy. So, a web application using XMLHttpRequestcould only make HTTP requests to its own domain. To improve web applications, developers asked browser vendors to allow XMLHttpRequest to make cross-domain requests.

The W3C Web Applications Working Group recommends the new Cross-Origin Resource Sharing(CORS) mechanism. CORS gives web servers cross-domain access controls, which enable secure cross-domain data transfers. Modern browsers use CORS in an API container - such as XMLHttpRequest - to mitigate risks of cross-origin HTTP requests.

This article is for web administrators, server developers and front-end developers. Modern browsers handle the client-side components of cross-origin sharing, including headers and policy enforcement. But this new standard means servers have to handle new request and response headers. Another article for server developers discussing cross-origin sharing from a server perspective (with PHP code snippets) is supplementary reading. 

This cross-origin sharing standard is used to enable cross-site HTTP requests for:

• Invocations of the 

  XMLHttpRequest

   API in a cross-site manner, as discussed above.
• Web Fonts (for cross-domain font usage in 

  @font-face

   within CSS), so that servers can deploy TrueType fonts that can only be cross-site loaded and used by web sites that are permitted to do so.
• WebGL textures.
• Images drawn to a canvas using drawImage.

This article is a general discussion of Cross-Origin Resource Sharing, and includes a discussion of the HTTP headers as implemented in Firefox 3.5. 

Overview

The Cross-Origin Resource Sharing standard works by adding new HTTP headers that allow servers to describe the set of origins that are permitted to read that information using a web browser.  Additionally, for HTTP request methods that can cause side-effects on user data (in particular, for 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.

Subsequent sections discuss scenarios, as well as a breakdown of the HTTP headers used. 

Examples of access control scenarios

Here, we present three scenarios that illustrate how Cross-Origin Resource Sharing works.  All of these examples use the 

XMLHttpRequest

 object, which can be used to make cross-site invocations in any supporting browser.

The JavaScript snippets included in these sections (and running instances of the server-code that correctly handles these cross-site requests) can be found "in action" here, and will work in browsers that support cross-site 

XMLHttpRequest

.  A discussion of Cross-Origin Resource Sharing from a server perspective (including PHP code snippets) can be found here.

Simple requests

A simple cross-site request is one that:

• Only uses GET, HEAD or POST. If POST is used to send data to the server, the Content-Type of the data sent to the server with the HTTP POST request is one of 

  application/x-www-form-urlencoded

  , 

  multipart/form-data

  , or 

  text/plain.

• Does not set custom headers with the HTTP Request (such as 

  X-Modified

  , etc.)

Note: These are the same kinds of cross-site requests that web content can already issue, and no response data is released to the requester unless the server sends an appropriate header. Therefore, sites that prevent cross-site request forgery have nothing new to fear from HTTP access control.

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(); 
  }
}           

Let us look at what the browser will send 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 by Firefox 3.5.  Note that 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 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 be 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:

• It uses methods other than 

  GET, HEAD

   or 

  POST

  .  Also, if 

  POST

   is used to send request data with a Content-Type other than 

  application/x-www-form-urlencoded

  , 

  multipart/form-data

  , or

  text/plain

  , e.g. if the 

  POST

   request sends an XML payload to the server using 

  application/xml

  or 

  text/xml

  , then the request is preflighted.
• It sets custom headers in the request (e.g. the request uses a header such as 

  X-PINGOTHER

  )

Note: Starting in Gecko 2.0, the 

text/plain

, 

application/x-www-form-urlencoded

, and 

multipart/form-data

data encodings can all be sent cross-site without preflighting. Previously, only 

text/plain

 could be sent without preflighting.

An example of this kind of invocation might be:

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 (

POST

) uses a Content-Type of 

application/xml

, and since a custom header is set, this request is preflighted.

Let's take a look at the full exchange between client and server:

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


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
Access-Control-Max-Age: 1728000
Vary: Accept-Encoding, Origin
Content-Encoding: gzip
Content-Length: 0
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive
Content-Type: text/plain

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.  Firefox 3.1 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 an idempotent 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 11 and 12 respectively):

Access-Control-Request-Method: POST
Access-Control-Request-Headers: X-PINGOTHER                

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 an

X-PINGOTHER

 custom header.  The server now has an opportunity to determine whether it wishes to accept a request under these circumstances.

Lines 15 - 27 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 18-21:

Access-Control-Allow-Origin: http://foo.example
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: X-PINGOTHER
Access-Control-Max-Age: 1728000                

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 HTTP/1.1 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

, confirming that this is a permitted header 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, 1728000 seconds is 20 days.

Requests with credentials

The most interesting capability exposed by both 

XMLHttpRequest

 and Access Control is the ability to make "credentialed" requests that are aware of HTTP Cookies and HTTP Authentication information.  By default, in cross-site 

XMLHttpRequest

 invocations, browsers will not send credentials.  A specific flag has to be set on the 

XMLHttpRequest

 object 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.  Important note: when responding to a credentialed request,  server must specify a domain, and cannot use wild carding.  The above example would fail if the header was wildcarded as: 

Access-Control-Allow-Origin: *

.  Since the 

Access-Control-Allow-Origin

explicitly mentions 

http://foo.example

, the credential-cognizant content is returned to the invoking web content.  Note that in line 22, a further cookie is set.

All of these examples can be seen working here.  The next section deals with the actual HTTP headers.

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.com to access the resource, you can specify:

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

If the server specifies an origin host rather than "*", then it must 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

(Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1)

This 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

This 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

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 | false                

Credentialed requests are discussed above.

Access-Control-Allow-Methods

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

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-origing sharing request headers programmatically.

Origin

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: The 

origin

 can be the empty string; this is useful, for example, if the source is a 

data

 URL.

Note that in any access control request, the 

ORIGIN

 header is always sent.

Access-Control-Request-Method

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

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.