Fix Strict SameSite Cookies on Cross-Domain Requests

G5 Cyber Security

2 weeks ago

TL;DR

Your browser is blocking a cookie because of its SameSite=Strict attribute when making requests to another domain. This happens for security reasons, preventing cross-site request forgery (CSRF) attacks. You need to understand how cookies and SameSite attributes work, then adjust your server or client code accordingly.

Understanding the Problem

Cookies are small pieces of data websites store on a user’s computer. They’re used for things like remembering login details or tracking items in a shopping cart. The SameSite attribute tells the browser when to send that cookie with requests.

Strict: Only send the cookie if the request originates from the same domain as the cookie was set.
Lax: Send the cookie for top-level navigations (e.g., clicking a link) and GET requests, but not for cross-site POST requests. This is often a good default.
None: Send the cookie with all requests, regardless of domain. Requires Secure attribute to be set as well (HTTPS only).

When your browser sees SameSite=Strict and you try to send that cookie on a request to a different domain, it blocks it for security.

Solution Steps

Identify the Cookie: Use your browser’s developer tools (usually by pressing F12) to find out which cookie is causing the issue. Look in the ‘Application’ or ‘Storage’ tab, under ‘Cookies’. Note the domain and name of the cookie.
- In Chrome DevTools, go to Application -> Cookies. Find your domain and inspect the cookies listed there.
Server-Side Fix (Recommended): The best solution is usually to adjust how the cookie is set on your server.
- If you control both domains, consider setting SameSite=Lax for most cookies. This provides a good balance of security and functionality.
- If you *need* to send the cookie across domains (e.g., for a single sign-on system), set SameSite=None and ensure the Secure attribute is also set.
```
Set-Cookie: mycookie=value; SameSite=None; Secure
```
- The specific way to do this depends on your server technology (e.g., PHP, Python/Flask, Node.js/Express). Here are some examples:
  - PHP: setcookie('mycookie', 'value', ['samesite' => 'None', 'secure' => true]);
  - Node.js (using Express): res.cookie('mycookie', 'value', { sameSite: 'None', secure: true });
Client-Side Fix (Less Recommended): If you can’t control the server, you might be able to work around it on the client side, but this is generally not ideal.
- Avoid making cross-domain requests that require the cookie. This may involve restructuring your application or using a different approach for authentication/authorization.
- If absolutely necessary and you understand the security implications, you could try to manually manage cookies using JavaScript (e.g., setting them only when needed), but this is complex and error-prone.
```
document.cookie = 'mycookie=value; path=/; domain=.example.com; samesite=None; secure';
```
Testing: After making changes, clear your browser’s cookies and cache to ensure the new settings are applied.
- Test thoroughly in different browsers (Chrome, Firefox, Safari) as cookie handling can vary.

Important Security Notes

HTTPS is essential: When using SameSite=None, the Secure attribute *must* be set to ensure cookies are only sent over HTTPS connections. Otherwise, your application is vulnerable to attacks.
CSRF Protection: Understand and implement proper CSRF protection mechanisms on both domains if you’re sharing cookies across them. This might involve using tokens or other security measures.