How does Access-Control-Allow-Origin header work?

Access-Control-Allow-Origin is a CORS (Cross-Origin Resource Sharing) header.

When Site A tries to fetch content from Site B, Site B can send an Access-Control-Allow-Origin response header to tell the browser that the content of this page is accessible to certain origins. (An origin is a domain, plus a scheme and port number.) By default, Site B's pages are not accessible to any other origin; using the Access-Control-Allow-Origin header opens a door for cross-origin access by specific requesting origins.

For each resource/page that Site B wants to make accessible to Site A, Site B should serve its pages with the response header:

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

Modern browsers will not block cross-domain requests outright. If Site A requests a page from Site B, the browser will actually fetch the requested page on the network level and check if the response headers list Site A as a permitted requester domain. If Site B has not indicated that Site A is allowed to access this page, the browser will trigger the XMLHttpRequest's error event and deny the response data to the requesting JavaScript code.

Non-simple requests

What happens on the network level can be slightly more complex than explained above. If the request is a "non-simple" request, the browser first sends a data-less "preflight" OPTIONS request, to verify that the server will accept the request. A request is non-simple when either (or both):

using an HTTP verb other than GET or POST (e.g. PUT, DELETE)

using non-simple request headers; the only simple requests headers are: Accept Accept-Language Content-Language Content-Type (this is only simple when its value is application/x-www-form-urlencoded, multipart/form-data, or text/plain)

Accept

Accept-Language

Content-Language

Content-Type (this is only simple when its value is application/x-www-form-urlencoded, multipart/form-data, or text/plain)

If the server responds to the OPTIONS preflight with appropriate response headers (Access-Control-Allow-Headers for non-simple headers, Access-Control-Allow-Methods for non-simple verbs) that match the non-simple verb and/or non-simple headers, then the browser sends the actual request.

Supposing that Site A wants to send a PUT request for /somePage, with a non-simple Content-Type value of application/json, the browser would first send a preflight request:

OPTIONS /somePage HTTP/1.1
Origin: http://siteA.com
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: Content-Type

Note that Access-Control-Request-Method and Access-Control-Request-Headers are added by the browser automatically; you do not need to add them. This OPTIONS preflight gets the successful response headers:

Access-Control-Allow-Origin: http://siteA.com
Access-Control-Allow-Methods: GET, POST, PUT
Access-Control-Allow-Headers: Content-Type

When sending the actual request (after preflight is done), the behavior is identical to how a simple request is handled. In other words, a non-simple request whose preflight is successful is treated the same as a simple request (i.e., the server must still send Access-Control-Allow-Origin again for the actual response).

The browsers sends the actual request:

PUT /somePage HTTP/1.1
Origin: http://siteA.com
Content-Type: application/json

{ "myRequestContent": "JSON is so great" }

And the server sends back an Access-Control-Allow-Origin, just as it would for a simple request:

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

See Understanding XMLHttpRequest over CORS for a little more information about non-simple requests.

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Cross-Origin Resource Sharing - CORS (A.K.A. Cross-Domain AJAX request) is an issue that most web developers might encounter, according to Same-Origin-Policy, browsers restrict client JavaScript in a security sandbox, usually JS cannot directly communicate with a remote server from a different domain. In the past developers created many tricky ways to achieve Cross-Domain resource request, most commonly using ways are:

Use Flash/Silverlight or server side as a "proxy" to communicate with remote. JSON With Padding (JSONP). Embeds remote server in an iframe and communicate through fragment or window.name, refer here.

Those tricky ways have more or less some issues, for example JSONP might result in security hole if developers simply "eval" it, and #3 above, although it works, both domains should build strict contract between each other, it neither flexible nor elegant IMHO:)

W3C had introduced Cross-Origin Resource Sharing (CORS) as a standard solution to provide a safe, flexible and a recommended standard way to solve this issue.

The Mechanism

From a high level we can simply deem CORS is a contract between client AJAX call from domain A and a page hosted on domain B, a typical Cross-Origin request/response would be:

DomainA AJAX request headers

Host DomainB.com
User-Agent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0) Gecko/20100101 Firefox/4.0
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8,application/json
Accept-Language en-us;
Accept-Encoding gzip, deflate
Keep-Alive 115
Origin http://DomainA.com 

DomainB response headers

Cache-Control private
Content-Type application/json; charset=utf-8
Access-Control-Allow-Origin DomainA.com
Content-Length 87
Proxy-Connection Keep-Alive
Connection Keep-Alive

The blue parts I marked above were the kernal facts, "Origin" request header "indicates where the cross-origin request or preflight request originates from", the "Access-Control-Allow-Origin" response header indicates this page allows remote request from DomainA (if the value is * indicate allows remote requests from any domain).

As I mentioned above, W3 recommended browser to implement a "preflight request" before submiting the actually Cross-Origin HTTP request, in a nutshell it is an HTTP OPTIONS request:

OPTIONS DomainB.com/foo.aspx HTTP/1.1

If foo.aspx supports OPTIONS HTTP verb, it might return response like below:

HTTP/1.1 200 OK
Date: Wed, 01 Mar 2011 15:38:19 GMT
Access-Control-Allow-Origin: http://DomainA.com
Access-Control-Allow-Methods: POST, GET, OPTIONS, HEAD
Access-Control-Allow-Headers: X-Requested-With
Access-Control-Max-Age: 1728000
Connection: Keep-Alive
Content-Type: application/json

Only if the response contains "Access-Control-Allow-Origin" AND its value is "*" or contain the domain who submitted the CORS request, by satisfying this mandtory condition browser will submit the actual Cross-Domain request, and cache the result in "Preflight-Result-Cache".

I blogged about CORS three years ago: AJAX Cross-Origin HTTP request

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Question is a bit too old to answer, but I am posting this for any future reference to this question.

According to this Mozilla Developer Network article,

A resource makes a cross-origin HTTP request when it requests a resource from a different domain, or port than the one which the first resource itself serves.

https://i.stack.imgur.com/zn3sT.png

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 (thus it should be cool).

Same-Origin Policy

For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts.
For example, XMLHttpRequest and Fetch follow the same-origin policy.
So, a web application using XMLHttpRequest or Fetch could only make HTTP requests to its own domain.

Cross-Origin Resource Sharing (CORS)

To improve web applications, developers asked browser vendors to allow cross-domain requests.

The Cross-Origin Resource Sharing (CORS) mechanism 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 or Fetch - to mitigate risks of cross-origin HTTP requests.

How CORS works (Access-Control-Allow-Origin header)

Wikipedia:

The CORS standard describes new HTTP headers which provide browsers and servers 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.

Example

The browser sends the OPTIONS request with an Origin HTTP header. The value of this header is the domain that served the parent page. When a page from http://www.example.com attempts to access a user's data in service.example.com, the following request header would be sent to service.example.com: Origin: http://www.example.com The server at service.example.com may respond with: An Access-Control-Allow-Origin (ACAO) header in its response indicating which origin sites are allowed. For example: Access-Control-Allow-Origin: http://www.example.com An error page if the server does not allow the cross-origin request An Access-Control-Allow-Origin (ACAO) header with a wildcard that allows all domains: Access-Control-Allow-Origin: *

---

Whenever I start thinking about CORS, my intuition about which site hosts the headers is incorrect, just as you described in your question. For me, it helps to think about the purpose of the same origin policy.

The purpose of the same origin policy is to protect you from malicious JavaScript on siteA.com accessing private information you've chosen to share only with siteB.com. Without the same origin policy, JavaScript written by the authors of siteA.com could make your browser make requests to siteB.com, using your authentication cookies for siteB.com. In this way, siteA.com could steal the secret information you share with siteB.com.

Sometimes you need to work cross domain, which is where CORS comes in. CORS relaxes the same origin policy for siteB.com, using the Access-Control-Allow-Origin header to list other domains (siteA.com) that are trusted to run JavaScript that can interact with siteB.com.

To understand which domain should serve the CORS headers, consider this. You visit malicious.com, which contains some JavaScript that tries to make a cross domain request to mybank.com. It should be up to mybank.com, not malicious.com, to decide whether or not it sets CORS headers that relax the same origin policy allowing the JavaScript from malicious.com to interact with it. If malicous.com could set its own CORS headers allowing its own JavaScript access to mybank.com, this would completely nullify the same origin policy.

I think the reason for my bad intuition is the point of view I have when developing a site. It's my site, with all my JavaScript, therefore it isn't doing anything malicious and it should be up to me to specify which other sites my JavaScript can interact with. When in fact I should be thinking which other sites JavaScript are trying to interact with my site and should I use CORS to allow them?

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From my own experience, it's hard to find a simple explanation why CORS is even a concern.

Once you understand why it's there, the headers and discussion becomes a lot clearer. I'll give it a shot in a few lines.

It's all about cookies. Cookies are stored on a client by their domain.

An example story: On your computer, there's a cookie for yourbank.com. Maybe your session is in there.

Key point: When a client makes a request to the server, it will send the cookies stored under the domain for that request.

You're logged in on your browser to yourbank.com. You request to see all your accounts, and cookies are sent for yourbank.com. yourbank.com receives the pile of cookies and sends back its response (your accounts).

If another client makes a cross origin request to a server, those cookies are sent along, just as before. Ruh roh.

You browse to malicious.com. Malicious makes a bunch of requests to different banks, including yourbank.com.

Since the cookies are validated as expected, the server will authorize the response.

Those cookies get gathered up and sent along - and now, malicious.com has a response from yourbank.

Yikes.

So now, a few questions and answers become apparent:

"Why don't we just block the browser from doing that?" Yep. CORS.

"How do we get around it?" Have the server tell the request that CORS is OK.

---

1. A client downloads javascript code MyCode.js from http://siteA - the origin.

The code that does the downloading - your html script tag or xhr from javascript or whatever - came from, let's say, http://siteZ. And, when the browser requests MyCode.js, it sends an Origin: header saying "Origin: http://siteZ", because it can see that you're requesting to siteA and siteZ != siteA. (You cannot stop or interfere with this.)

2. The response header of MyCode.js contains Access-Control-Allow-Origin: http://siteB, which I thought meant that MyCode.js was allowed to make cross-origin references to the site B.

no. It means, Only siteB is allowed to do this request. So your request for MyCode.js from siteZ gets an error instead, and the browser typically gives you nothing. But if you make your server return A-C-A-O: siteZ instead, you'll get MyCode.js . Or if it sends '*', that'll work, that'll let everybody in. Or if the server always sends the string from the Origin: header... but... for security, if you're afraid of hackers, your server should only allow origins on a shortlist, that are allowed to make those requests.

Then, MyCode.js comes from siteA. When it makes requests to siteB, they are all cross-origin, the browser sends Origin: siteA, and siteB has to take the siteA, recognize it's on the short list of allowed requesters, and send back A-C-A-O: siteA. Only then will the browser let your script get the result of those requests.

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Using React and Axios, join proxy link to the URL and add header as shown below

https://cors-anywhere.herokuapp.com/ + Your API URL

Just by adding the Proxy link will work, but it can also throw error for No Access again. Hence better to add header as shown below.

axios.get(`https://cors-anywhere.herokuapp.com/[YOUR_API_URL]`,{headers: {'Access-Control-Allow-Origin': '*'}})
      .then(response => console.log(response:data);
  }

WARNING: Not to be used in Production

This is just a quick fix, if you're struggling with why you're not able to get a response, you CAN use this. But again it's not the best answer for production. Got several downvotes and it completely makes sense, I should have added the warning a long time ago.

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If you want just to test a cross domain application in which the browser blocks your request, then you can just open your browser in unsafe mode and test your application without changing your code and without making your code unsafe. From MAC OS you can do this from the terminal line:

open -a Google\ Chrome --args --disable-web-security --user-data-dir

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If you are using PHP, try adding the following code at the beginning of the php file:

If you are using localhost, try this:

header("Access-Control-Allow-Origin: *");

If you are using external domains such as server, try this:

header("Access-Control-Allow-Origin: http://www.website.com");

---

I work with express 4 and node 7.4 and angular,I had the same problem me help this: a) server side: in file app.js I give headers to all response like:

app.use(function(req, res, next) {  
      res.header('Access-Control-Allow-Origin', req.headers.origin);
      res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
      next();
 });  

this must have before all router. I saw a lot of added this headers:

res.header("Access-Control-Allow-Headers","*");
res.header('Access-Control-Allow-Credentials', true);
res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');

but i dont need that, b) client side: in send ajax you need add: "withCredentials: true," like:

$http({
     method: 'POST',
     url: 'url, 
     withCredentials: true,
     data : {}
   }).then(function(response){
        // code  
   }, function (response) {
         // code 
   });

---

In Python I have been using the Flask-CORS library with great success. It makes dealing with CORS super easy and painless. I added some code from the library's documentation below.

Installing:

$ pip install -U flask-cors

Simple example that allows CORS for all domains on all routes:

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app)

@app.route("/")
def helloWorld():
  return "Hello, cross-origin-world!"

For more specific examples see the documentation. I have used the simple example above to get around the CORS issue in an ionic application I am building that has to access a separate flask server.

---

For cross origin sharing, set header: 'Access-Control-Allow-Origin':'*';

Php: header('Access-Control-Allow-Origin':'*');

Node: app.use('Access-Control-Allow-Origin':'*');

This will allow to share content for different domain.

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Simply paste the following code in your web.config file.

Noted that, you have to paste the following code under <system.webServer> tag

    <httpProtocol>  
    <customHeaders>  
     <add name="Access-Control-Allow-Origin" value="*" />  
     <add name="Access-Control-Allow-Headers" value="Content-Type" />  
     <add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE, OPTIONS" />  
    </customHeaders>  
  </httpProtocol>  

---

I can't configure it in back-end server but with these extensions in browsers work for me:

For Firefox: Cors Everywhere

For Google Chrome: Allow CORS: Access-Control-Allow-Origin

Note: CORS work for me with this configuration:

https://i.stack.imgur.com/eDD7P.jpg

https://i.stack.imgur.com/27QWb.jpg

---

Nginx and Appache

As addition to apsillers answer I would like to add wiki graph which shows when request is simple or not (and OPTIONS pre-flight request is send or not)

https://i.stack.imgur.com/BTFel.png

For simple request (e.g. hotlinking images) you don't need to change your server configuration files but you can add headers in application (hosted on server, e.g. in php) like Melvin Guerrero mention in his answer - but remember: if you add full cors headers in you server (config) and at same time you allow simple cors on application (e.g. php) this will not work at all.

And here are configurations for two popular servers

turn on CORS on Nginx (nginx.conf file) location ~ ^/index\.php(/|$) { ... add_header 'Access-Control-Allow-Origin' "$http_origin" always; # if you change "$http_origin" to "*" you shoud get same result - allow all domain to CORS (but better change it to your particular domain) add_header 'Access-Control-Allow-Credentials' 'true' always; if ($request_method = OPTIONS) { add_header 'Access-Control-Allow-Origin' "$http_origin"; # DO NOT remove THIS LINES (doubled with outside 'if' above) add_header 'Access-Control-Allow-Credentials' 'true'; add_header 'Access-Control-Max-Age' 1728000; # cache preflight value for 20 days add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS'; # arbitrary methods add_header 'Access-Control-Allow-Headers' 'My-First-Header,My-Second-Header,Authorization,Content-Type,Accept,Origin'; # arbitrary headers add_header 'Content-Length' 0; add_header 'Content-Type' 'text/plain charset=UTF-8'; return 204; } }

turn on CORS on Appache (.htaccess file) # ------------------------------------------------------------------------------ # | Cross-domain Ajax requests | # ------------------------------------------------------------------------------ # Enable cross-origin Ajax requests. # http://code.google.com/p/html5security/wiki/CrossOriginRequestSecurity # http://enable-cors.org/ # change * (allow any domain) below to your domain Header set Access-Control-Allow-Origin "*" Header always set Access-Control-Allow-Methods "POST, GET, OPTIONS, DELETE, PUT" Header always set Access-Control-Allow-Headers "My-First-Header,My-Second-Header,Authorization, content-type, csrf-token" Header always set Access-Control-Allow-Credentials "true"

---

The Access-Control-Allow-Origin response header indicates whether the response can be shared with requesting code from the given origin.

Header type Response       header
Forbidden header name      no

A response that tells the browser to allow code from any origin to access a resource will include the following:

Access-Control-Allow-Origin: *

For more info, visit here....

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ONLY TEMPORARY SOLUTION for Testing:

Who can't control backend for Options 405 Method Not Allowed.
Workaround for Chrome browser.
execute in command line:
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --disable-web-security --user-data-dir="path_to_profile"
Example:
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --disable-web-security --user-data-dir="C:\Users\vital\AppData\Local\Google\Chrome\User Data\Profile 2"

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For .NET Core 3.1 API With Angular

Startup.cs : Add CORS

    //SERVICES
    public void ConfigureServices(IServiceCollection services){

        //CORS (Cross Origin Resource Sharing)
        //=====================================
        services.AddCors();
    }

    //MIDDLEWARES
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.UseRouting();

        //ORDER: CORS -> Authentication -> Authorization)
        //CORS (Cross Origin Resource Sharing)
        //=====================================  
        app.UseCors(x=>x.AllowAnyHeader().AllowAnyMethod().WithOrigins("http://localhost:4200"));

        app.UseHttpsRedirection();
    }
}

Controller : Enable CORS For Authorized Controller

 //Authorize all methods inside this controller
 [Authorize]
 [EnableCors()]
 public class UsersController : ControllerBase
 {
    //ActionMethods
 }

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