Cross Domain Form POSTing

web-development

cross-domain

cors

csrf

byAlex Kataev·Sep 4, 2024

Cross Domain Form POSTing challenge is overcome by using CORS. Configure the server that responds to your POST request to include Access-Control-Allow-Origin header. As a server admin, include this in your configuration:

// Look mum, no CORS!
res.header("Access-Control-Allow-Origin", "*"); // '*' can be substituted with your domain

If you can't access the server, a server-side proxy is the way to go. This PHP snippet serves as a handy proxy, posting data to an external domain:

<?php
// My life as a proxy starts... now!
$context = stream_context_create(['http' => [
  'method'  => 'POST',
  'header'  => 'Content-Type: application/x-www-form-urlencoded',
  'content' => http_build_query($_POST)
]]);
// Houston, we have a POST!
echo file_get_contents('http://externaldomain.com/target', false, $context);
?>

Don't be tempted by client-side only solutions — browser security will catch you unless the target service supports JSONP or CORS. Treat security standards and privacy regulations like your in-laws: always accommodate them.

Inside the Same-origin Policy

The same-origin policy is your guiding light in navigating cross-domain POSTing. It gauges how a script or document from one origin interacts with resources from another origin. In short, it's the superhero of web security.

Dodging the Restrictions

Even superheroes have blind spots: the policy does not prevent POST requests to a different domain. Use the form's action attribute, set as a different server URL, as your cape to dodge these restrictions. But remember, when the response is not of the same origin or CORS is misconfigured, the returned message may become invisible.

Dealing with Security Tokens

Harness anti-forgery tokens or CSRF tokens to iron-clad your forms against cross-site request forgery (CSRF) attacks. This piece of magic ensures form submissions originate from your website and not from some masked villain.

Mastering CSRF Attacks

Here's your crash course: CSRF attacks occur through unauthorized form postings. The trickster-attacker poses as a trusted friend to the site and gets the user to perform unwanted actions. The catch? CSRF relies on side effects (like state changes) and not on reading data.

Contemplating on Client-Server Relations

Need to read the response?

A direct form POST may send a request, but might fail to get the response from a different domain due to browser restrictions.

Preserving Server-side Honesty

Ensure your server-side code is a trustworthy ally. It must handle values from different domains accurately and perform necessary validations and checks.

Safeguarding Privacy

Keep user data sacred! Understand the ethics and laws related to handling cross-domain data and ensure your data protection ritual is followed meticulously.

Paramedic Solutions and Coding Examples

The Back-end Proxying

Establish a server-side knight that receives the POST request and dutifully delivers it to the other domain. This knight can be your secret weapon in controlling requests and adhering to protocols.

Tread Carefully with JSONP

JSONP handshakes with the same-origin policy using a script tag. However, it can only process GET requests and lacks the secure framework of CORS.

Libraries like CORS Whisperers

Buzzwords like jQuery and modern APIs such as Fetch are your allies in processing CORS requests. They are equipped with compatibility functions that ease out browser incompatibility issues with headers.

explain-codes / Web development / Cross Domain Form POSTing

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