29 Temmuz 2019

Fetch: Cross-Origin Requests

If we make a fetch from an arbitrary web-site, that will probably fail.

The core concept here is origin – a domain/port/protocol triplet.

Cross-origin requests – those sent to another domain (even a subdomain) or protocol or port – require special headers from the remote side. That policy is called “CORS”: Cross-Origin Resource Sharing.

For instance, let’s try fetching http://example.com:

try {
  await fetch('http://example.com');
} catch(err) {
  alert(err); // Failed to fetch

Fetch fails, as expected.

Why? A brief history

Because cross-origin restrictions protect the internet from evil hackers.

Seriously. Let’s make a very brief historical digression.

For many years a script from one site could not access the content of another site.

That simple, yet powerful rule was a foundation of the internet security. E.g. a script from the page hacker.com could not access user’s mailbox at gmail.com. People felt safe.

JavaScript also did not have any special methods to perform network requests at that time. It was a toy language to decorate a web page.

But web developers demanded more power. A variety of tricks were invented to work around the limitation.

Using forms

One way to communicate with another server was to submit a <form> there. People submitted it into <iframe>, just to stay on the current page, like this:

<!-- form target -->
<iframe name="iframe"></iframe>

<!-- a form could be dynamically generated and submited by JavaScript -->
<form target="iframe" method="POST" action="http://another.com/…">

So, it was possible to make a GET/POST request to another site, even without networking methods. But as it’s forbidden to access the content of an <iframe> from another site, it wasn’t possible to read the response.

…Okay, in fact there actually were tricks for that (required special scripts at both remote and our page), but let’s not delve deeper. Nothing good in those for us now.

Using scripts

Another trick was to use a <script src="http://another.com/…"> tag. A script could have any src, from any domain. But again – it was impossible to access the raw content of such script.

If another.com intended to expose data for this kind of access, then a so-called “JSONP (JSON with padding)” protocol was used.

Let’s say we need to get the data from http://another.com this way:

  1. First, in advance, we declare a global function to accept the data, e.g. gotWeather.

    // 1. Declare the function to process the data
    function gotWeather({ temperature, humidity }) {
      alert(`temperature: ${temperature}, humidity: ${humidity}`);
  2. Then we make a <script> tag with src="http://another.com/weather.json?callback=gotWeather", please note that the name of our function is its callback parameter.

    let script = document.createElement('script');
    script.src = `http://another.com/weather.json?callback=gotWeather`;
  3. The remote server dynamically generates a script that calls gotWeather(...) with the data it wants us to receive.

    // The expected answer from the server looks like this:
      temperature: 25,
      humidity: 78
  4. As the script executes, gotWeather runs, and, as it’s our function, we have the data.

That works, and doesn’t violate security, because both sides agreed to pass the data this way. And, when both sides agree, it’s definitely not a hack. There are still services that provide such access, as it works even for very old browsers.

After a while, networking methods appeared, such as XMLHttpRequest.

At first, cross-origin requests were forbidden. But as a result of long discussions, cross-domain requests were allowed, in a way that does not add any capabilities unless explicitly allowed by the server.

Simple requests

There are two types of cross-domain requests:

  1. Simple requests.
  2. All the others.

Simple Requests are, well, simpler to make, so let’s start with them.

A simple request is a request that satisfies two conditions:

  1. Simple method: GET, POST or HEAD
  2. Simple headers – the only allowed custom headers are:
    • Accept,
    • Accept-Language,
    • Content-Language,
    • Content-Type with the value application/x-www-form-urlencoded, multipart/form-data or text/plain.

Any other request is considered “non-simple”. For instance, a request with PUT method or with an API-Key HTTP-header does not fit the limitations.

The essential difference is that a “simple request” can be made with a <form> or a <script>, without any special methods.

So, even a very old server should be ready to accept a simple request.

Contrary to that, requests with non-standard headers or e.g. method DELETE can’t be created this way. For a long time JavaScript was unable to do such requests. So an old server may assume that such requests come from a privileged source, “because a webpage is unable to send them”.

When we try to make a non-simple request, the browser sends a special “preflight” request that asks the server – does it agree to accept such cross-origin requests, or not?

And, unless the server explicitly confirms that with headers, a non-simple request is not sent.

Now we’ll go into details. All of them serve a single purpose – to ensure that new cross-origin capabilities are only accessible with an explicit permission from the server.

CORS for simple requests

If a request is cross-origin, the browser always adds Origin header to it.

For instance, if we request https://anywhere.com/request from https://javascript.info/page, the headers will be like:

GET /request
Host: anywhere.com
Origin: https://javascript.info

As you can see, Origin contains exactly the origin (domain/protocol/port), without a path.

The server can inspect the Origin and, if it agrees to accept such a request, adds a special header Access-Control-Allow-Origin to the response. That header should contain the allowed origin (in our case https://javascript.info), or a star *. Then the response is successful, otherwise an error.

The browser plays the role of a trusted mediator here:

  1. It ensures that the corrent Origin is sent with a cross-domain request.
  2. If checks for correct Access-Control-Allow-Origin in the response, if it is so, then JavaScript access, otherwise forbids with an error.

Here’s an example of an “accepting” response:

200 OK
Content-Type:text/html; charset=UTF-8
Access-Control-Allow-Origin: https://javascript.info

Response headers

For cross-origin request, by default JavaScript may only access “simple response headers”:

  • Cache-Control
  • Content-Language
  • Content-Type
  • Expires
  • Last-Modified
  • Pragma

Any other response header is forbidden.

Please note: no Content-Length

Please note: there’s no Content-Length header in the list!

So, if we’re downloading something and would like to track the percentage of progress, then an additional permission is required to access that header (see below).

To grant JavaScript access to any other response header, the server must list it in the Access-Control-Expose-Headers header.

For example:

200 OK
Content-Type:text/html; charset=UTF-8
Content-Length: 12345
API-Key: 2c9de507f2c54aa1
Access-Control-Allow-Origin: https://javascript.info
Access-Control-Expose-Headers: Content-Length,API-Key

With such Access-Control-Expose-Headers header, the script is allowed to access Content-Length and API-Key headers of the response.

“Non-simple” requests

We can use any HTTP-method: not just GET/POST, but also PATCH, DELETE and others.

Some time ago no one could even assume that a webpage is able to do such requests. So there may exist webservices that treat a non-standard method as a signal: “That’s not a browser”. They can take it into account when checking access rights.

So, to avoid misunderstandings, any “non-simple” request – that couldn’t be done in the old times, the browser does not make such requests right away. Before it sends a preliminary, so-called “preflight” request, asking for permission.

A preflight request uses method OPTIONS and has no body.

  • Access-Control-Request-Method header has the requested method.
  • Access-Control-Request-Headers header provides a comma-separated list of non-simple HTTP-headers.

If the server agrees to serve the requests, then it should respond with status 200, without body.

  • The response header Access-Control-Allow-Methods must have the allowed method.
  • The response header Access-Control-Allow-Headers must have a list of allowed headers.
  • Additionally, the header Access-Control-Max-Age may specify a number of seconds to cache the permissions. So the browser won’t have to send a preflight for subsequent requests that satisfy given permissions.

Let’s see how it works step-by-step on example, for a cross-domain PATCH request (this method is often used to update data):

let response = await fetch('https://site.com/service.json', {
  method: 'PATCH',
  headers: {
    'Content-Type': 'application/json'
    'API-Key': 'secret'

There are three reasons why the request is not simple (one is enough):

  • Method PATCH
  • Content-Type is not one of: application/x-www-form-urlencoded, multipart/form-data, text/plain.
  • Custom API-Key header.

Step 1 (preflight request)

The browser, on its own, sends a preflight request that looks like this:

OPTIONS /service.json
Host: site.com
Origin: https://javascript.info
Access-Control-Request-Method: PATCH
Access-Control-Request-Headers: Content-Type,API-Key
  • Method: OPTIONS.
  • The path – exactly the same as the main request: /service.json.
  • Cross-origin special headers:
    • Origin – the source origin.
    • Access-Control-Request-Method – requested method.
    • Access-Control-Request-Headers – a comma-separated list of “non-simple” headers.

Step 2 (preflight response)

The server should respond with status 200 and headers:

  • Access-Control-Allow-Methods: PATCH
  • Access-Control-Allow-Headers: Content-Type,API-Key.

That would allow future communication, otherwise an error is triggered.

If the server expects other methods and headers, makes sense to list them all at once, e.g:

200 OK
Access-Control-Allow-Methods: PUT,PATCH,DELETE
Access-Control-Allow-Headers: API-Key,Content-Type,If-Modified-Since,Cache-Control
Access-Control-Max-Age: 86400

Now the browser can see that PATCH is in the list of allowed methods, and both headers are in the list too, so it sends out the main request.

Besides, the preflight response is cached for time, specified by Access-Control-Max-Age header (86400 seconds, one day), so subsequent requests will not cause a preflight. Assuming that they fit the allowances, they will be sent directly.

Step 3 (actual request)

When the preflight is successful, the browser now makes the real request. Here the flow is the same as for simple requests.

The real request has Origin header (because it’s cross-origin):

PATCH /service.json
Host: site.com
Content-Type: application/json
API-Key: secret
Origin: https://javascript.info

Step 4 (actual response)

The server should not forget to add Access-Control-Allow-Origin to the response. A successful preflight does not relieve from that:

Access-Control-Allow-Origin: https://javascript.info

Now everything’s correct. JavaScript is able to read the full response.


A cross-origin request by default does not bring any credentials (cookies or HTTP authentication).

That’s uncommon for HTTP-requests. Usually, a request to http://site.com is accompanied by all cookies from that domain. But cross-domain requests made by JavaScript methods are an exception.

For example, fetch('http://another.com') does not send any cookies, even those that belong to another.com domain.


That’s because a request with credentials is much more powerful than an anonymous one. If allowed, it grants JavaScript the full power to act and access sensitive information on behalf of a user.

Does the server really trust pages from Origin that much? A request with credentials needs an additional header to pass through.

To enable credentials, we need to add the option credentials: "include", like this:

fetch('http://another.com', {
  credentials: "include"

Now fetch sends cookies originating from another.com with the request.

If the server wishes to accept the request with credentials, it should add a header Access-Control-Allow-Credentials: true to the response, in addition to Access-Control-Allow-Origin.

For example:

200 OK
Access-Control-Allow-Origin: https://javascript.info
Access-Control-Allow-Credentials: true

Please note: Access-Control-Allow-Origin is prohibited from using a star * for requests with credentials. There must be exactly the origin there, like above. That’s an additional safety measure, to ensure that the server really knows who it trusts.


Networking methods split cross-origin requests into two kinds: “simple” and all the others.

Simple requests must satisfy the following conditions:

  • Method: GET, POST or HEAD.
  • Headers – we can set only:
    • Accept
    • Accept-Language
    • Content-Language
    • Content-Type to the value application/x-www-form-urlencoded, multipart/form-data or text/plain.

The essential difference is that simple requests were doable since ancient times using <form> or <script> tags, while non-simple were impossible for browsers for a long time.

So, practical difference is that simple requests are sent right away, with Origin header, but for other ones the browser makes a preliminary “preflight” request, asking for permission.

For simple requests:

  • → The browser sends Origin header with the origin.
  • ← For requests without credentials (default), the server should set:
    • Access-Control-Allow-Origin to * or same as Origin
  • ← For requests with credentials, the server should set:
    • Access-Control-Allow-Origin to Origin
    • Access-Control-Allow-Credentials to true

Additionally, if JavaScript wants to access non-simple response headers:

  • Cache-Control
  • Content-Language
  • Content-Type
  • Expires
  • Last-Modified
  • Pragma

…Then the server should list the allowed ones in Access-Control-Expose-Headers header.

For non-simple requests, a preliminary “preflight” request is issued before the requested one:

  • → The browser sends OPTIONS request to the same url, with headers:
    • Access-Control-Request-Method has requested method.
    • Access-Control-Request-Headers lists non-simple requested headers
  • ← The server should respond with status 200 and headers:
    • Access-Control-Allow-Methods with a list of allowed methods,
    • Access-Control-Allow-Headers with a list of allowed headers,
    • Access-Control-Max-Age with a number of seconds to cache permissions.
  • Then the actual request is sent, the previous “simple” scheme is applied.


önem: 5

As you probably know, there’s HTTP-header Referer, that usually contains an url of the page which initiated a network request.

For instance, when fetching http://google.com from http://javascript.info/some/url, the headers look like this:

Accept: */*
Accept-Charset: utf-8
Accept-Encoding: gzip,deflate,sdch
Connection: keep-alive
Host: google.com
Origin: http://javascript.info
Referer: http://javascript.info/some/url

As you can see, both Referer and Origin are present.

The questions:

  1. Why Origin is needed, if Referer has even more information?
  2. If it possible that there’s no Referer or Origin, or it’s incorrect?

We need Origin, because sometimes Referer is absent. For instance, when we fetch HTTP-page from HTTPS (access less secure from more secure), then there’s no Referer.

The Content Security Policy may forbid sending a Referer.

As we’ll see, fetch also has options that prevent sending the Referer and even allow to change it (within the same site).

By specification, Referer is an optional HTTP-header.

Exactly because Referer is unreliable, Origin was invented. The browser guarantees correct Origin for cross-origin requests.

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