By Peter David Carter


2016-02-22 12:19:25 8 Comments

tl;dr; About the Same Origin Policy

I have a Grunt process which initiates an instance of express.js server. This was working absolutely fine up until just now when it started serving a blank page with the following appearing in the error log in the developer's console in Chrome (latest version):

XMLHttpRequest cannot load https://www.example.com/ No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:4300' is therefore not allowed access.

What is stopping me from accessing the page?

6 comments

@Quentin 2016-02-22 12:26:59

tl;dr — There's a summary at the end and headings in the answer to make it easier to find the relevant parts. Reading everything is recommended though as it provides useful background for understanding the why that makes seeing how the how applies in different circumstances easier.

About the Same Origin Policy

This is the Same Origin Policy. It is a security feature implemented by browsers.

Your particular case is showing how it is implemented for XMLHttpRequest (and you'll get identical results if you were to use fetch), but it also applies to other things (such as images loaded onto a <canvas> or documents loaded into an <iframe>), just with slightly different implementations.

(Weirdly, it also applies to CSS fonts, but that is because found foundries insisted on DRM and not for the security issues that the Same Origin Policy usually covers).

The standard scenario that demonstrates the need for the SOP can be demonstrated with three characters:

  • Alice is a person with a web browser
  • Bob runs a website (https://www.[website].com/ in your example)
  • Mallory runs a website (http://localhost:4300 in your example)

Alice is logged into Bob's site and has some confidential data there. Perhaps it is a company intranet (accessible only to browsers on the LAN), or her online banking (accessible only with a cookie you get after entering a username and password).

Alice visits Mallory's website which has some JavaScript that causes Alice's browser to make an HTTP request to Bob's website (from her IP address with her cookies, etc). This could be as simple as using XMLHttpRequest and reading the responseText.

The browser's Same Origin Policy prevents that JavaScript from reading the data returned by Bob's website (which Bob and Alice don't want Mallory to access). (Note that you can, for example, display an image using an <img> element across origins because the content of the image is not exposed to JavaScript (or Mallory) … unless you throw canvas into the mix in which case you will generate a same-origin violation error).


Why the Same Origin Policy applies when you don't think it should

For any given URL it is possible that the SOP is not needed. A couple of common scenarios where this is the case are:

  • Alice, Bob and Mallory are the same person.
  • Bob is providing entirely public information

… but the browser has no way of knowing if either of the above are true, so trust is not automatic and the SOP is applied. Permission has to be granted explicitly before the browser will give the data it was given to a different website.


Why the Same Origin Policy only applies to JavaScript in a web page

Browser extensions, the Network tab in browser developer tools and applications like Postman are installed software. They aren't passing data from one website to the JavaScript belonging to a different website just because you visited that different website. Installing software usually takes a more conscious choice.

There isn't a third party (Mallory) who is considered a risk.


Why you can display data in the page without reading it with JS

There are a number of circumstances where Mallory's site can cause a browser to fetch data from a third party and display it (e.g. by adding an <img> element to display an image). It isn't possible for Mallory's JavaScript to read the data in that resource though, only Alice's browser and Bob's server can do that, so it is still secure.


CORS

The Access-Control-Allow-Origin header referred to in the error message is part of the CORS standard which allows Bob to explicitly grant permission to Mallory's site to access the data via Alice's browser.

A basic implementation would just include:

Access-Control-Allow-Origin: *

… to permit any website to read the data.

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

… would allow only a specific site to access it, and Bob can dynamically generate that based on the Origin request header to permit multiple, but not all, sites to access it.

The specifics of how Bob sets that response header depend on Bob's HTTP server and/or server-side programming language. There is a collection of guides for various common configurations that might help.

Model of where CORS rules are applied

NB: Some requests are complex and send a preflight OPTIONS request that the server will have to respond to before the browser will send the GET/POST/PUT/Whatever request that the JS wants to make. Implementations of CORS that only add Access-Control-Allow-Origin to specific URLs often get tripped up by this.


Obviously granting permission via CORS is something Bob would only do only if either:

  • The data was not private or
  • Mallory was trusted

But I'm not Bob!

There is no standard mechanism for Mallory to add this header because it has to come from Bob's website, which she does not control.

If Bob is running a public API then there might be a mechanism to turn on CORS (perhaps by formatting the request in a certain way, or a config option after logging into a Developer Portal site for Bob's site). This will have to be a mechanism implemented by Bob though. Mallory could read the documentation on Bob's site to see if something is available, or she could talk to Bob and ask him to implement CORS.


Error messages which mention "Response for preflight"

Some cross origin requests are preflighted.

This happens when (roughly speaking) you try to make a cross-origin request that:

  • Includes credentials like cookies
  • Couldn't be generated with a regular HTML form (e.g. has custom headers or a Content-Type that you couldn't use in a form's enctype).

If you are correctly doing something that needs a preflight

In these cases then the rest of this answer still applies but you also need to make sure that the server can listen for the preflight request (which will be OPTIONS (and not GET, POST or whatever you were trying to send) and respond to it with the right Access-Control-Allow-Origin header but also Access-Control-Allow-Methods and Access-Control-Allow-Headers to allow your specific HTTP methods or headers.

If you are triggering a preflight by mistake

Sometimes people make mistakes when trying to construct Ajax requests, and sometimes these trigger the need for a preflight. If the API is designed to allow cross-origin requests, but doesn't require anything that would need a preflight, then this can break access.

Common mistakes that trigger this include:

  • trying to put Access-Control-Allow-Origin and other CORS response headers on the request. These don't belong on the request, don't do anything helpful (what would be the point of a permissions system where you could grant yourself permission?), and must appear only on the response.
  • trying to put a Content-Type: application/json header on a GET request that has no request body to describe the content of (typically when the author confuses Content-Type and Accept).

In either of these cases, removing the extra request header will often be enough to avoid the need for a preflight (which will solve the problem when communicating with APIs that support simple requests but not preflighted requests).


Opaque responses

Sometimes you need to make an HTTP request, but you don't need to read the response. e.g. if you are posting a log message to the server for recording.

If you are using the fetch API (rather than XMLHttpRequest), then you can configure it to not try to use CORS.

Note that this won't let you do anything that you require CORS to do. You will not be able to read the response. You will not be able to make a request that requires a preflight.

It will let you make a simple request, not see the response, and not fill the Developer Console with error messages.

How to do it is explained by the Chrome error message given when you make a request using fetch and don't get permission to view the response with CORS:

Access to fetch at 'https://example.com/' from origin 'https://example.net' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

Thus:

fetch("http://example.com", { mode: "no-cors" });

Alternatives to CORS

JSONP

Bob could also provide the data using a hack like JSONP which is how people did cross-origin Ajax before CORS came along.

It works by presenting the data in the form of a JavaScript program which injects the data into Mallory's page.

It requires that Mallory trust Bob not to provide malicious code.

Note the common theme: The site providing the data has to tell the browser that it is OK for a third party site to access the data it is sending to the browser.

Since JSONP works by appending a <script> element to load the data in the form of a JavaScript program which calls a function already in the page, attempting to use the JSONP technique on a URL which returns JSON will fail — typically with a CORB error — because JSON is not JavaScript.

Move the two resources to a single Origin

If the HTML document the JS runs in and the URL being requested are on the same origin (sharing the same scheme, hostname, and port) then they Same Origin Policy grants permission by default. CORS is not needed.

A Proxy

Mallory could use server-side code to fetch the data (which she could then pass from her server to Alice's browser through HTTP as usual).

It will either:

  • add CORS headers
  • convert the response to JSONP
  • exist on the same origin as the HTML document

That server-side code could be written & hosted by a third party (such as CORS Anywhere). Note the privacy implications of this: The third party can monitor who proxies what across their servers.

Bob wouldn't need to grant any permissions for that to happen.

This would be fine since that is just between Mallory and Bob. There is no way for Bob to think that Mallory is Alice and to provide Mallory with data that should be kept confidential between Alice and Bob.

Consequently, Mallory can only use this technique to read public data.

Writing something other than a web app

As noted in the section "Why the Same Origin Policy only applies to JavaScript in a web page", you can avoid the SOP by not writing JavaScript in a webpage.

That doesn't mean you can't continue to use JavaScript and HTML, but you could distribute it using some other mechanism, such as Node-WebKit or PhoneGap.

Browser extensions

It is possible for a browser extension to inject the CORS headers in the response before the Same Origin Policy is applied.

These can be useful for development, but are not practical for a production site (asking every user of your site to install a browser extension that disables a security feature of their browser is unreasonable).

They also tend to work only with simple requests (failing when handling preflight OPTIONS requests).

Having a proper development environment with a local development server is usually a better approach.


Other security risks

Note that SOP / CORS do not mitigate XSS, CSRF, or SQL Injection attacks which need to be handled independently.


Summary

  • There is nothing you can do in your client-side code that will enable CORS access to someone else's server.
  • If you control the server the request is being made to: Add CORS permissions to it.
  • If you are friendly with the person who controls it: Get them to add CORS permissions to it.
  • If it is a public service:
    • Read their API documentation to see what they say about accessing it with client-side JavaScript:
      • They might tell you to use specific URLs
      • They might support JSONP
      • They might not support cross-origin access from client-side code at all (this might be a deliberate decision on security grounds, especially if you have to pass a personalised API Key in each request).
    • Make sure you aren't triggering a preflight request you don't need. The API might grant permission for simple requests but not preflighted requests.
  • If none of the above apply: Get the browser to talk to your server instead, and then have your server fetch the data from the other server and pass it on. (There are also third-party hosted services which attach CORS headers to publically accessible resources that you could use).

@Ciasto piekarz 2016-12-23 05:09:49

If I run local LAN a web server and try to do ajax load from the IP/URL will that work ? I havent tried that yet. as my web server returing json data would be a MCU

@Quentin 2016-12-23 12:25:58

@Ciastopiekarz — Normal same origin / different origin rules apply. Normal network routing rules apply.

@pungggi 2017-04-22 18:12:08

Most complete answer I ve ever read, instead of just a link about cors..

@snippetkid 2017-05-11 11:51:38

@Quentin - Wow! +1! So what I'm to understand is if Alice uses the CORS extension, the server thinks that her http calls are not from javascript but from a browser extension and treats it like a normal same origin request?

@Quentin 2017-05-11 11:54:04

@snippetkid — No. In the usual case, the server will send CORS headers in ever response and not care where the request came from. It is the responsibility of the browser to allow or deny access to the data to the JS based on the CORS headers on the response. (Things get a /little/ more complex on the server when it comes to preflight requests)

@Suraj Jain 2018-04-18 02:43:45

'what language you are using for server-side programming (if any).' Could you clear this up for me? Server Side Programming will require a language.

@Quentin 2018-04-18 08:01:22

@SurajJain — You might choose to add the headers by configuring your HTTP server and no do any programming at all.

@Suraj Jain 2018-05-06 10:08:41

@Quentin This answer on SOP in security stackexchange was wrong according to me, so I edited it, could you please verify the edit, I learned about SOP from your answers and some other domains and am not sure about if I wrote correct or not.

@Cromon 2018-09-15 10:43:12

So I cannot edit OPs question and he apparently refuses to add any suggestions to it, but the reason it provides benefits even when proxies are involved is with cookies. Even when the proxy is completely transparent Alices browser (assuming it works properly) wont send (Bobs) cookies to Mallorys proxy and therefore Bobs server has no reason to believe that he is talking to Alice. Subsequently it wont send any info related to Alice to the proxy operated by Mallory. - Just in case anyone was wondering like me and forgot about cookies :)

@Sergei Kovalenko 2019-10-10 09:39:29

shouldn't be the third person's name Cecil?

@Quentin 2019-10-10 10:08:07

@Sergei Kovalenko 2019-10-14 08:50:18

@Quentin, Ah, that's why. Thanks!

@zwif 2017-11-09 11:37:49

As this isn't mentioned in the accepted answer.

  • This is not the case for this exact question, but might help others that search for that problem
  • This is something you can do in your client-code to prevent CORS errors in some cases.

You can make use of Simple Requests.
In order to perform a 'Simple Requests' the request needs to meet several conditions. E.g. only allowing POST, GET and HEAD method, as well as only allowing some given Headers (you can find all conditions here).

If your client code does not explicit set affected Headers (e.g. "Accept") with a fix value in the request it might occur that some clients do set these Headers automatically with some "non-standard" values causing the server to not accept it as Simple Request - which will give you a CORS error.

@morph85 2018-12-17 06:40:45

This CORS issue wasn't further elaborated (for other causes).

I'm having this issue currently under different reason. My front end is returning 'Access-Control-Allow-Origin' header error as well.

Just that I've pointed the wrong URL so this header wasn't reflected properly (in which i kept presume it did). localhost (front end) -> call to non secured http (supposed to be https), make sure the API end point from front end is pointing to the correct protocol.

@Perostek Balveda 2017-04-17 11:13:49

You should enable CORS to get it working.

@Daphoque 2016-09-09 10:45:32

Target server must allowed cross-origin request. In order to allow it through express, simply handle http options request :

app.options('/url...', function(req, res, next){
   res.header('Access-Control-Allow-Origin', "*");
   res.header('Access-Control-Allow-Methods', 'POST');
   res.header("Access-Control-Allow-Headers", "accept, content-type");
   res.header("Access-Control-Max-Age", "1728000");
   return res.sendStatus(200);
});

@Vishnu 2016-02-22 12:32:38

This is happening because of the CORS error. CORS stands for Cross Origin Resource Sharing. In simple words, this error occurs when we try to access a domain/resource from another domain.

Read More about it here: CORS error with jquery

To fix this, if you have access to the other domain, you will have to allow Access-Control-Allow-Origin in the server. This can be added in the headers. You can enable this for all the requests/domains or a specific domain.

How to get a cross-origin resource sharing (CORS) post request working

These links may help

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