CORS not working in Angular?

CORS not working in Angular? - javascript

I am trying to GET information from this site
https://bitcoinindex.es/api/v0.1/coinbase/usd/btc/last
Using the $http service
After looking all over the internet Here is my code in coffeescript
angular.module('blackmoonApp')
.controller 'PricingCtrl', ($scope, $http) ->
$http.defaults.useXDomain = true
$http.get("https://bitcoinindex.es/api/v0.1/coinbase/usd/btc/last",
headers:
"Access-Control-Allow-Origin": "*"
).success (JSON) ->
console.log JSON
The Result is
"XMLHttpRequest cannot load https://www.bitstamp.net/api/ticker/. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:9000' is therefore not allowed access."
I am not sure if the website is blocking me (which wouldn't make sense because it is an API) or if AngularJS isn't able to work with CORS.

Access-Control-Allow-Origin is a response header.
This:
headers:
"Access-Control-Allow-Origin": "*"
… sets a request header.
You need to set it on https://www.bitstamp.net/api/ticker/, not in your JavaScript.
It would defeat the object if any JavaScript could grant itself permission to access any server.
I am not sure if the website is blocking me (which wouldn't make sense because it is an API)
Blocking is the default behaviour. Explicit permission must be granted to allow JavaScript from other origins access. Otherwise anyone with a bitstamp account could have their bitcoins stolen by visiting a website that used the API (since it would be their browser, with their cookies, making the request).

From your code:
$http.get("https://www.bitstamp.net/api/ticker/",
headers:
"Access-Control-Allow-Origin": "*"
)
We see that you are attempting to send the Access-Control-Allow-Origin header in the GET request.
CORS doesn't work that way; the Access-Control-Allow-Origin header must be present in the response sent by the server (and, of course, such header must include the exact same domain of the page that sent the request).
From my tests (unlikely, but your results may differ):
https://bitcoinindex.es/api/v0.1/coinbase/usd/btc/last only allows CORS requests from its own domain (the response had Access-Control-Allow-Origin:https://bitcoinindex.es) which is kind of pointless; and
https://www.bitstamp.net/api/ticker/ didn't have the Access-Control-Allow-Origin header at all.

Related

CORS blocked origin - Origin and allowed origin are the same

Using dockerized Ory Kratos and Angular (www folder hosted via nginx to be able to modify headers) on localhost and trying to execute
const headers = {
Accept: 'application/json',
};
fetch('http://127.0.0.1:4433/self-service/registration/browser', {
method: 'GET',
headers,
redirect: 'follow',
credentials: 'include',
})
.then((r) => console.log(r))
.catch((f) => console.log(f));
leads to
Access to fetch at 'http://127.0.0.1:8100/auth/register?flow=b35c3f9a-5592-4121-80b9-87503c38e1d3' (redirected from 'http://127.0.0.1:4433/self-service/registration/browser') from origin 'http://127.0.0.1:8100' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header has a value 'http://127.0.0.1:8100' that is not equal to the supplied origin. Have the server send the header with a valid value, or, if an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
http://127.0.0.1:4433/self-service/registration/browser redirects to http://127.0.0.1:8100/custompath/register?flow=[some-flow-id] via a 302 HTTP response. The browser tries to resolve the redirect and throws the error mentioned above.
Origin and allowed origin are the same - so how can that error happen? I already found this answer on stackoverflow: https://stackoverflow.com/a/62320912/14345380 if that's helpful.
Chrome version 87.0.4280.88
Edit #1
Network tab output can be seen
here
Edit #2
To be able to resolve the issue yourself I set up a little repository: Find it here
Edit #3
Thanks for the hint from pandamakes. The test repo is not running on :8100 but on 4200 (Switched from ionic project to angular project).
Edit #4
I started another discussion at the ory/kratos project origin here. The Ory Team implemented a SDK which we can use instead of plain fetch requests.

One thing is simply add no-cors to the request.
mode: 'no-cors'
Also, you say you have Access-Control-Allow-Origin header set did you also set Access-Control-Allow-Credentials: true , since you are fetching with credentials: "include".
Lastly setting credentials: "include" origin can not be a localhost or 127.0.0.1 or it will return false and cause a cors error, so either, set credentials to same-site or omit or get yourself a domain name.

I ... think I got it...
The 302 redirection works, but when the fetch requests follows the redirection, it now attempts at fetching a non-cors resource. In this event, it will not attach an Origin request header (since it is a non CORS request).
When the response arrives, the browser will compare access-control-allow-origin header, if any exist, to the origin of the request. If the response header exists, and is not *, it must match the origin header of the request.
So solve your issue, you will want to remove the access-control-allow-origin in your nginx setup, or set it to *
min repro: https://github.com/xgui3783/cors-min-repro

How to solve CORS Origin issue when trying to get data of a json file from localhost

I was trying to get the data from a json file (which is on the remote server) from localhost and I am getting error 'from origin 'http://localhost:5000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.'
Below is the code I used to send the get request.
$.ajax({
url: 'http://abc/some.json',
type: "GET",
dataType: "json",
headers: {
"Access-Control-Request-Headers": "*",
"Access-Control-Request-Method": "*",
'Content-Type':'application/json'
},
success: function (data) {
alert(data);
chatbotConfig = data;
console.log(chatbotConfig);
}
})
I am able to get the data using postman. I understand that postman is not a browser so it is not limited by CORS policy. But I am able to download the content from json file directly from the browser. Nit able to understand why I am facing issue when I tried to access from localhost. Can any one please help me to resolve this issue.

Well, the CORS error may lie in different layers of your application. Usually, by allowing all (*) origins and headers in the server-side, your problem should be fixed. But sometimes it won't and it lies somewhere else like your HTTP request header or body, DNS settings (Making sure there are no third-party DNS set in your network by yourself through the OS), or browser settings. So I will describe all of the possible solutions here shortly.
NOTE: You can read more about CORS policy here.
How to solve CORS error
If you have access to your server-side settings and/or options you need to make sure (Or if you don't you need to ask your backend developer) that everything set correctly there or not. So the steps will be as follows:
First of all, you need to make sure your server configs for CORS are set correctly.
So for example, if you are using Apache you can set the CORS policy in your .htaccess file like this (or simply check if it exists):
// This will allow all origins to access your server
Header set Access-Control-Allow-Origin "*" // You can set * to whatever domain
Header add Access-Control-Allow-Methods "*"
If you are using nodeJS you can check or set it like this:
app.use((req, res, next) => {
res.setHeader("Access-Control-Allow-Origin", "*");
res.header(
"Access-Control-Allow-Headers",
"Origin, X-Requested-With, Content-Type, Accept"
);
next();
});
If you are using Nginx you can check or set it in nginx.conf like this:
server {
location / {
add_header Access-Control-Allow-Origin "*";
add_header Access-Control-Allow-Methods "*";
}
}
If you use any other service for your server-side you should read the documentation about it, but all of them are actually same.
If you checked the above step and CORS policy was set correctly in your server-side, you may need to check your HTTP request and make sure everything is set correctly there. For example, there is no need to add these two headers
"Access-Control-Request-Headers": "*",
"Access-Control-Request-Method": "*",
in your request, because they do not exist as a header in the client-side. Wrong HTTP request API end-point address or options (like dataType), wrong HTTP request body or headers may also cause the CORS error for your here. So you need to check them all and make sure you are set them correctly.
If none of the above was the problem you can try two other approaches. I mostly encourage to use the second one if none of the above solves your problem.
Mapping your localhost to a URL using your hosts file (Usually lies under etc folder in each operating system). You can do that like this (But it's better to read the documentation for each OS).
127.0.0.1 www.test.com ## map 127.0.0.1 to www.test.com
## or
localhost www.test.com
NOTE: Usually it is better to avoid this approach.
Use browser plugins like Allow CORS.

Basically if you clearly look into the issue logs:
'from origin 'http://localhost:5000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource
It's saying CORS not enabled at resource. which means it need to be fixed or allowed at server side.

javascript: fetch blocked by CORS policy because of wildcard

I am trying to use the following api endpoint using fetch:
https://api.guerrillamail.com/ajax.php?f=check_email&ip=${ip}&agent=${agent}
(outdated documentation)
When I set credentials: 'include' I get the following error:
Access to fetch at 'https://api.guerrillamail.com/ajax.php?...' has been blocked by CORS policy: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'.
I have to set the flag in order to submit a cookie for authentication.
After googling this problem for 1++ hour, my understanding is the following:
CORS has to be server side allowed to be able to make "none simple" request to an other domain e.g. localhost => guerrillamail.com to prevent abuse, a variable Access-Control-Allow-Origin has to be set to the domains that should be allowed to send requests. A valid option is "*", which means that ALL origins are ok.
For some reason it is not ok though in combination with the credentials: 'include flag.
Do you have any ideas why this wouldnt be allowed?
Do you know what I have to do, in order to to do the request?
And is my understanding about this correct?

The documentation you're referencing no longer applies. In that old documentation, the API was made available over HTTP rather than HTTPS. CORS doesn't apply to HTTP and wouldn't have been a problem.
In the latest documention, that API is provided over HTTPS. To deal with the CORS requirement, they also removed the need for cookies, changing it to subscr_token and sid_token parameters sent as part of the request:
version 1.5, 30th May 2011
- Removed the requirement for cookies, added subscr_token and sid_token parameters

Fetch API: No 'Access-Control-Allow-Origin' error even after adding "Access-Control-Allow-Origin": "*" to my headers [duplicate]

Apparently, I have completely misunderstood its semantics. I thought of something like this:
A client downloads JavaScript code MyCode.js from http://siteA - the origin.
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.
The client triggers some functionality of MyCode.js, which in turn make requests to http://siteB, which should be fine, despite being cross-origin requests.
Well, I am wrong. It does not work like this at all. So, I have read Cross-origin resource sharing and attempted to read Cross-Origin Resource Sharing in w3c recommendation.
One thing is sure - I still do not understand how I am supposed to use this header.
I have full control of both site A and site B. How do I enable the JavaScript code downloaded from the site A to access resources on the site B using this header?
P.S.: I do not want to utilize JSONP.

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)
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.

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 as 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

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.
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 style sheets, 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 have 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?

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. That's 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.

Using React and Axios, join a proxy link to the URL and add a header as shown below:
https://cors-anywhere.herokuapp.com/ + Your API URL
Just adding the proxy link will work, but it can also throw an error for No Access again. Hence it is better to add a 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.

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 worked with Express.js 4, Node.js 7.4 and Angular, and I had the same problem. This helped me:
a) server side: in file app.js I add headers to all responses, 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 be before all routes.
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 don’t need that,
b) client side: in sending by Ajax, you need to add "withCredentials: true," like:
$http({
method: 'POST',
url: 'url',
withCredentials: true,
data : {}
}).then(function(response){
// Code
}, function (response) {
// Code
});

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 macOS, you can do this from the terminal line:
open -a Google\ Chrome --args --disable-web-security --user-data-dir

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.

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 on the back-end server, but with these extensions in the browsers, it works for me:
For Firefox:
CORS Everywhere
For Google Chrome:
Allow CORS: Access-Control-Allow-Origin
Note: CORS works for me with this configuration:

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.

Nginx and Apache
As an addition to apsiller's answer, I would like to add a wiki graph which shows when a request is simple or not (and OPTIONS pre-flight request is send or not)
For a simple request (e.g., hotlinking images), you don't need to change your server configuration files, but you can add headers in the application (hosted on the server, e.g., in PHP) like Melvin Guerrero mentions in his answer - but remember: if you add full CORS headers in your server (configuration) and at same time you allow simple CORS in the 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 Apache (.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 information, visit Access-Control-Allow-Origin...

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
}

Note: Only a temporary solution for testing
For those who can't control the backend for Options 405 Method Not Allowed, here is a workaround for theChrome browser.
Execute in the 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"

Most CORS issues are because you are trying to request via client side ajax from a react, angular, jquery apps that are frontend basic libs.
You must request from a backend application.
You are trying to request from a frontend API, but the API you are trying to consume is expecting this request to be made from a backend application and it will never accept client side requests.

ajax request error "access control allow origin" [duplicate]

Apparently, I have completely misunderstood its semantics. I thought of something like this:
A client downloads JavaScript code MyCode.js from http://siteA - the origin.
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.
The client triggers some functionality of MyCode.js, which in turn make requests to http://siteB, which should be fine, despite being cross-origin requests.
Well, I am wrong. It does not work like this at all. So, I have read Cross-origin resource sharing and attempted to read Cross-Origin Resource Sharing in w3c recommendation.
One thing is sure - I still do not understand how I am supposed to use this header.
I have full control of both site A and site B. How do I enable the JavaScript code downloaded from the site A to access resources on the site B using this header?
P.S.: I do not want to utilize JSONP.

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)
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.

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 as 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

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. That's 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.

Using React and Axios, join a proxy link to the URL and add a header as shown below:
https://cors-anywhere.herokuapp.com/ + Your API URL
Just adding the proxy link will work, but it can also throw an error for No Access again. Hence it is better to add a 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.

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 worked with Express.js 4, Node.js 7.4 and Angular, and I had the same problem. This helped me:
a) server side: in file app.js I add headers to all responses, 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 be before all routes.
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 don’t need that,
b) client side: in sending by Ajax, you need to add "withCredentials: true," like:
$http({
method: 'POST',
url: 'url',
withCredentials: true,
data : {}
}).then(function(response){
// Code
}, function (response) {
// Code
});

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 macOS, you can do this from the terminal line:
open -a Google\ Chrome --args --disable-web-security --user-data-dir

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.

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 on the back-end server, but with these extensions in the browsers, it works for me:
For Firefox:
CORS Everywhere
For Google Chrome:
Allow CORS: Access-Control-Allow-Origin
Note: CORS works for me with this configuration:

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.

Nginx and Apache
As an addition to apsiller's answer, I would like to add a wiki graph which shows when a request is simple or not (and OPTIONS pre-flight request is send or not)
For a simple request (e.g., hotlinking images), you don't need to change your server configuration files, but you can add headers in the application (hosted on the server, e.g., in PHP) like Melvin Guerrero mentions in his answer - but remember: if you add full CORS headers in your server (configuration) and at same time you allow simple CORS in the 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 Apache (.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 information, visit Access-Control-Allow-Origin...

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
}

Note: Only a temporary solution for testing
For those who can't control the backend for Options 405 Method Not Allowed, here is a workaround for theChrome browser.
Execute in the 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"

Most CORS issues are because you are trying to request via client side ajax from a react, angular, jquery apps that are frontend basic libs.
You must request from a backend application.
You are trying to request from a frontend API, but the API you are trying to consume is expecting this request to be made from a backend application and it will never accept client side requests.

Develop Reference

JavaScript is the programming language of the Web.