SameSite Lax Bypass via CSRF Method Override!

SameSite Lax Bypass Explained (Simple & Practical)​

Many web apps depend on default browser behavior and assume CSRF protection is enabled automatically. One common mistake is not explicitly setting the SameSite cookie attribute.
When this happens, modern browsers apply SameSite=Lax by default - and that can be abused .

With the help of HTTP Method Override, attackers may bypass CSRF protections and trigger sensitive actions without user consent.

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The Core Idea Behind SameSite=Lax​

When a website does not define SameSite in the response headers:

• Browsers treat the cookie as SameSite=Lax
• SameSite=Lax allows cookies to be sent with top-level GET requests
• If the backend trusts method override parameters, a POST action can be executed via GET

This creates a dangerous CSRF scenario.

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Step-by-Step Attack Methodology​

Step 1: Capture a Sensitive Request​

Intercept a request that performs a critical action, such as:

• Email change
• Password update

In most cases, this request uses the POST method.

Step 2: Convert POST to GET​

Now comes the trick :

• Change the request method from POST to GET
• Add a method override parameter to the URL: &_method=POST

This fools the backend into processing the request as a POST request.

Step 3: Send the Request​

If the server responds with 200 OK and the action succeeds, the endpoint is vulnerable to CSRF under SameSite=Lax behavior .

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CSRF Proof of Concept (PoC)​

Here’s a simple JavaScript example to demonstrate the attack:

document.location = "https://target.com/account/change-email?email=attacker@email.com&_method=POST";

Once the victim visits this link, the browser automatically attaches session cookies.

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Why This Attack Works​

This bypass succeeds because:

• The request is a top-level GET navigation
• SameSite=Lax allows cookies to be sent
• The backend trusts the method override parameter
• Session cookies are included automatically
• The sensitive action executes without user interaction

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Key Security Takeaways​

Always test for method override parameters
Never trust default SameSite behavior
Do not assume CSRF protection just because the request uses POST
Backend logic matters more than frontend assumptions
Happy hunting and secure coding!