Mod note: This question is about why XMLHttpRequest/fetch/etc. on the browser are subject to the Same Access Policy restrictions (you get errors mentioning CORB or CORS) while Postman is not. This question is not about how to fix a "No 'Access-Control-Allow-Origin'..." error. It's about why they happen.

Please stop posting:

  • CORS configurations for every language/framework under the sun. Instead find your relevant language/framework's question.
  • 3rd party services that allow a request to circumvent CORS
  • Command line options for turning off CORS for various browsers

I am trying to do authorization using JavaScript by connecting to the RESTful API built-in Flask. However, when I make the request, I get the following error:

XMLHttpRequest cannot load http://myApiUrl/login. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access.

I know that the API or remote resource must set the header, but why did it work when I made the request via the Chrome extension Postman?

This is the request code:

$.ajax({
  type: 'POST',
  dataType: 'text',
  url: api,
  username: 'user',
  password: 'pass',
  crossDomain: true,
  xhrFields: {
    withCredentials: true,
  },
})
  .done(function (data) {
    console.log('done');
  })
  .fail(function (xhr, textStatus, errorThrown) {
    alert(xhr.responseText);
    alert(textStatus);
  });
javascript jquery cors same-origin-policy flask-restless 
4 Answers

If I understood it right you are doing an XMLHttpRequest to a different domain than your page is on. So the browser is blocking it as it usually allows a request in the same origin for security reasons. You need to do something different when you want to do a cross-domain request. A tutorial about how to achieve that is Using CORS.

When you are using Postman they are not restricted by this policy. Quoted from Cross-Origin XMLHttpRequest:

Regular web pages can use the XMLHttpRequest object to send and receive data from remote servers, but they're limited by the same origin policy. Extensions aren't so limited. An extension can talk to remote servers outside of its origin, as long as it first requests cross-origin permissions.

answered Jan 28


Applying a CORS restriction is a security feature defined by a server and implemented by a browser.

The browser looks at the CORS policy of the server and respects it.

However, the Postman tool does not bother about the CORS policy of the server.

That is why the CORS error appears in the browser, but not in Postman.

answered Jan 28


You might also get this error if your gateway timeout is too short and the resource you are accessing takes longer to process than the timeout. This may be the case for complex database queries etc. Thus, the above error code can be disguishing this problem. Just check if the error code is 504 instead of 404 as in Kamils answer above or something else. If it is 504, then increasing the gateway timeout might fix the problem.

In my case the CORS error could be removed by disabling the same origin policy (CORS) in the IE browser, see How to disable same origin policy Internet Explorer. After doing this it was a pure 504 error in the log.

answered Jan 28


It's very simple to solve if you are using PHP. Just add the following script in the beginning of your PHP page which handles the request:

<?php header('Access-Control-Allow-Origin: *'); ?>

If you are using Node-red you have to allow CORS in the node-red/settings.js file by un-commenting the following lines:

httpNodeCors: {
 origin: "*",
 methods: "GET,PUT,POST,DELETE"
},

If you are using Flask same as the question; you have first to install flask-cors

$ pip install -U flask-cors

Then include the Flask cors in your application.

from flask_cors import CORS

A simple application will look like:

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app)

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

Hope it helps!

You can also ask if you have some more queries related to javascript or you can join our Java training class and ask the top instructors by yourself.

answered Jan 28


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