SameSite Cookie Attribute: What It Is and Why It Matters

The SameSite attribute is a cookie attribute that controls whether a cookie is sent with cross-site requests. It helps protect against Cross-Site Request Forgery (CSRF) attacks by allowing developers to specify when cookies should be sent in a cross-origin context. The SameSite attribute can have three possible values: Strict, Lax, and None.

1. SameSite=Strict

• Behavior: Cookies with the SameSite=Strict attribute are only sent in requests that originate from the same site that set the cookie. This means the cookie is not sent along with requests initiated by third-party websites, even if a link or a form on a different site triggers a request to the original site.
• Use Case: Use SameSite=Strict when you want to ensure that the cookie is only sent in a first-party context. This provides the highest level of protection against CSRF attacks but may limit functionality in some cases (e.g., when users navigate to your site via links from other sites).

Example:

Set-Cookie: session_id=abc123; SameSite=Strict

2. SameSite=Lax

• Behavior: Cookies with the SameSite=Lax attribute are sent with requests that originate from the same site, as well as with “safe” cross-site requests like top-level navigation (e.g., when the user clicks a link to your site) or when a form is submitted. However, the cookie is not sent with less safe requests like those triggered by third-party scripts (e.g., iframes, AJAX requests).
• Use Case: Use SameSite=Lax when you want to allow cookies to be sent with most user-initiated navigations but still want some protection against CSRF attacks. This is a good balance between security and usability for most websites.

Example:

Set-Cookie: session_id=abc123; SameSite=Lax

3. SameSite=None

• Behavior: Cookies with the SameSite=None attribute are sent with all requests, including cross-site requests. However, to use SameSite=None, the cookie must also be marked as Secure, meaning it will only be sent over HTTPS.
• Use Case: Use SameSite=None when your application requires the cookie to be sent with all cross-site requests. This is common for third-party content providers, like social media widgets or analytics services, that need to track user sessions across different sites.

Example:

Set-Cookie: session_id=abc123; SameSite=None; Secure

Important Notes:

• Browser Defaults: If the SameSite attribute is not explicitly set, modern browsers may default to SameSite=Lax to enhance security.
• Legacy Support: Older browsers may not recognize the SameSite attribute. For these browsers, cookies will behave as if the attribute is not set.