Can Cross-Domain Cookies Be Set, and If Not, What Are the Alternatives?

Cross-Domain Cookies: Setting a Cookie for Another Domain

Setting cookies for a different domain than the one on which the user's current session originated may seem straightforward. However, this action is not possible due to security concerns.

Browsers implement a same-origin policy that prevents cookies set by one domain from being sent along with requests to another domain. When a cookie is set by a.com, for example, it can only be included in subsequent requests to a.com.

Why Cross-Domain Cookies Are Prohibited

Allowing cross-domain cookies would pose a significant security risk. Malicious websites could exploit this vulnerability to steal session cookies, passwords, and other sensitive information from a user's session on a different domain.

Alternative Approach

If you need to set a cookie for b.com from a.com, you can request b.com to set the cookie on its own. This can be achieved by redirecting the user to a custom URL on b.com, where the cookie is set and the user is then redirected to the desired destination.

An example of such a script on b.com could be:

<?php
    setcookie('a', $_GET['c']);
    header("Location: b.com/landingpage.php");
?>

This script sets the 'a' cookie to the value provided in the 'c' GET parameter and then redirects the user to the 'landingpage.php' page on b.com.

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