Fetch: Cross-Origin Requests

If we send a fetch request to another web-site, it will probably fail.

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.

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.

Why is CORS needed? A brief history

CORS exists to 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. an evil script from website hacker.com could not access the user’s mailbox at website 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 and make requests to other websites.

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/…">
  ...
</form>

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

To be precise, there were actually tricks for that, they required special scripts at both the iframe and the page. So the communication with the iframe was technically possible. Right now there’s no point to go into details, let these dinosaurs rest in peace.

Using scripts

Another trick was to use a script tag. A script could have any src, with any domain, like <script src="http://another.com/…">. It’s possible to execute a script from any website.

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

Here’s how it worked.

Let’s say we, at our site, need to get the data from http://another.com, such as the weather:

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

   // 1. Declare the function to process the weather 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", using the name of our function as the callback URL-parameter.

   let script = document.createElement('script');
   script.src = `http://another.com/weather.json?callback=gotWeather`;
   document.body.append(script);

3. The remote server another.com dynamically generates a script that calls gotWeather(...) with the data it wants us to receive.

   // The expected answer from the server looks like this:
   gotWeather({
     temperature: 25,
     humidity: 78
   });

4. When the remote script loads and 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 in browser JavaScript.

At first, cross-origin requests were forbidden. But as a result of long discussions, cross-origin requests were allowed, but with any new capabilities requiring an explicit allowance by the server, expressed in special headers.

Safe requests

There are two types of cross-origin requests:

1. Safe requests.
2. All the others.

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

A request is safe if it satisfies two conditions:

1. Safe method: GET, POST or HEAD
2. Safe 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 “unsafe”. 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 safe 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 safe 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 unsafe 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, an unsafe request is not sent.

Now we’ll go into details.

CORS for safe requests

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

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

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

As you can see, the Origin header 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, add 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 it’s an error.

The browser plays the role of a trusted mediator here:

1. It ensures that the correct Origin is sent with a cross-origin request.
2. It checks for permitting Access-Control-Allow-Origin in the response, if it exists, then JavaScript is allowed to access the response, otherwise it fails with an error.

Here’s an example of a permissive server 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 so-called “safe” response headers:

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

Accessing any other response header causes an error.

To grant JavaScript access to any other response header, the server must send the Access-Control-Expose-Headers header. It contains a comma-separated list of unsafe header names that should be made accessible.

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

“Unsafe” requests

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

Some time ago no one could even imagine that a webpage could make such requests. So there may still 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 “unsafe” request – that couldn’t be done in the old times, the browser does not make such requests right away. First, it sends a preliminary, so-called “preflight” request, to ask for permission.

A preflight request uses the method OPTIONS, no body and two headers:

• Access-Control-Request-Method header has the method of the unsafe request.
• Access-Control-Request-Headers header provides a comma-separated list of its unsafe HTTP-headers.

If the server agrees to serve the requests, then it should respond with empty body, status 200 and headers:

• Access-Control-Allow-Origin must be either * or the requesting origin, such as https://javascript.info, to allow it.
• Access-Control-Allow-Methods must have the allowed method.
• 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 the example of a cross-origin 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 unsafe (one is enough):

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

Step 1 (preflight request)

Prior to sending such a 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 “unsafe” headers.

Step 2 (preflight response)

The server should respond with status 200 and the headers:

• Access-Control-Allow-Origin: https://javascript.info
• Access-Control-Allow-Methods: PATCH
• Access-Control-Allow-Headers: Content-Type,API-Key.

That allows future communication, otherwise an error is triggered.

If the server expects other methods and headers in the future, it makes sense to allow them in advance by adding them to the list.

For example, this response also allows PUT, DELETE and additional headers:

200 OK
Access-Control-Allow-Origin: https://javascript.info
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 Access-Control-Allow-Methods and Content-Type,API-Key are in the list Access-Control-Allow-Headers, so it sends out the main request.

If there’s the header Access-Control-Max-Age with a number of seconds, then the preflight permissions are cached for the given time. The response above will be cached for 86400 seconds (one day). Within this timeframe, subsequent requests will not cause a preflight. Assuming that they fit the cached allowances, they will be sent directly.

Step 3 (actual request)

When the preflight is successful, the browser now makes the main request. The process here is the same as for safe requests.

The main request has the 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 main response. A successful preflight does not relieve from that:

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

Then JavaScript is able to read the main server response.

Credentials

A cross-origin request initiated by JavaScript code 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. Cross-origin requests made by JavaScript methods on the other hand are an exception.

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

Why?

That’s because a request with credentials is much more powerful than without them. If allowed, it grants JavaScript the full power to act on behalf of the user and access sensitive information using their credentials.

Does the server really trust the script that much? Then it must explicitly allow requests with credentials with an additional header.

To send credentials in fetch, 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 request to that site.

If the server agrees 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. Like shown above, it must provide the exact origin there. That’s an additional safety measure, to ensure that the server really knows who it trusts to make such requests.

Summary

From the browser point of view, there are two kinds of cross-origin requests: “safe” and all the others.

“Safe” 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 safe requests were doable since ancient times using <form> or <script> tags, while unsafe were impossible for browsers for a long time.

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

For safe requests:

• → The browser sends the Origin header with the origin.
• ← For requests without credentials (not sent by default), the server should set:
  • Access-Control-Allow-Origin to * or same value as Origin
• ← For requests with credentials, the server should set:
  • Access-Control-Allow-Origin to same value as Origin
  • Access-Control-Allow-Credentials to true

Additionally, to grant JavaScript access to any response headers except Cache-Control, Content-Language, Content-Type, Expires, Last-Modified or Pragma, the server should list the allowed ones in Access-Control-Expose-Headers header.

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

• → The browser sends an OPTIONS request to the same URL, with the headers:
  • Access-Control-Request-Method has requested method.
  • Access-Control-Request-Headers lists unsafe requested headers.
• ← The server should respond with status 200 and the 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 the permissions.
• Then the actual request is sent, and the previous “safe” scheme is applied.