Next-Router-State-Tree

[zeaiter-squared]

Summary

How exactly does the Next-Router-State-Tree cookie get used on the server? Is it possible to disable this cookie entirely?

A project I am working on will have different responses when the request is intercepted a value in Next-Router-State-Tree is changed to differ from the actual request path.

Additional information

No response

Example

No response

[KarkiGaurav]

Hi the Next-Router-State-Tree cookie is basically something Next.js uses internally to keep the client router state in sync with the server. It stores the current router tree so the server knows which parts of the page to re-render during navigation or RSC updates.

It does not affect your app logic directly.
If someone edits the value manually, Next.js usually ignores it because the structure does not match what the framework expects.

As of now, this cookie cannot be disabled.
Even if you remove it in middleware, Next.js will recreate it again, and some app-router features like soft navigation or partial rendering may stop working.

If you need to make routing decisions, it is better to rely on the actual req.nextUrl or your own custom cookie instead of reading or modifying this internal one.

[Umuts-Codes]

Hi zeaiter-squared,

The Next-Router-State-Tree cookie is used internally by Next.js to manage server-side routing state in the App Router, particularly for server components and dynamic routing. It keeps track of the router state between requests and ensures that server-rendered content aligns with the expected navigation path.

How it is used on the server

1. Server-Side Routing
   When a request hits the server, Next.js reads this cookie to reconstruct the router state. This allows the server to correctly render components that depend on the current route, dynamic segments, or nested layouts.

2. Hydration Consistency
   The cookie ensures that the state on the client matches what was rendered on the server. Without it, server-side rendered pages may fail to hydrate correctly, leading to mismatches in component state or navigation issues.

• Disabling the cookie -

Currently, Next.js does not provide a built-in option to fully disable the Next-Router-State-Tree cookie. It is an integral part of the App Router’s mechanism for managing dynamic routes and server component state.

• Recommended approach -

• If you need to manipulate or intercept the routing state, consider using server-side logic to inspect or modify requests based on route parameters rather than disabling the cookie entirely.
• For projects concerned about cookie overhead, note that the cookie is minimal in size and scoped to the routing state, so it generally has negligible impact on performance.

• Summary -

• The cookie is essential for correct server-side rendering and hydration in Next.js App Router.
• It tracks dynamic routing and server component state.
• Fully disabling it is not supported; instead, adapt server logic to account for or safely modify its values if needed.

[zeaiter-squared]

This cookie popped up on a penetration test for our teams application. In particular, it identified changes in data being served to the client when Next-Router-State-Tree was intercepted and manipulated.

Would this fall under what is considered expected behaviour for the application & cookie? Would it be sensible to use middleware to make sure that Next-Router-State-Tree only has valid values?

[Umuts-Codes]

Hi,

The behavior you observed during the penetration test—changes in data when Next-Router-State-Tree was intercepted—would not be considered typical expected behavior. This cookie is primarily used by Next.js internally to track server-side routing state and ensure client-server hydration consistency. Manipulating it on the client can lead to mismatched or unexpected content being rendered.

Using middleware to validate or sanitize the values of Next-Router-State-Tree before they reach server-side logic is a sensible approach. This helps ensure that only valid routing state is processed, mitigating potential client-side manipulation risks. Server-side verification of critical route-dependent data is recommended whenever possible.

Overall, the cookie itself should not be disabled, as it is essential for the App Router and dynamic routing. Instead, focus on safe server-side handling and validation if your application requires stricter control over client-provided values.

Happy to provide example implementations if needed!

[zeaiter-squared]

Sorry, allow me to clarify because reading back my earlier reply it seems I may have been a bit misleading.

In our particular case the issue arose on a profile page. Naturally, on such a page you will show certain data from a user DTO. This page is also dynamically rendered as well. Now, when a field on Next-Router-State-Tree is changed to something arbitrary, the page will of course still render properly, however inside the http reponse from the server there will be the user DTO object that is used to construct the profile page.

I suppose as a dynamically rendered page it is possible that the user DTO is sent with the request for something that may be getting rendered on the client, but does such a thing make sense to occur when Next-Router-State-Tree is manipulated? Would sending a user DTO to the client be useful to "reset" the client side of the application?

[Umuts-Codes]

Hi zeaiter-squared,

No problem for miselading. Based on your description, the behavior you observed is consistent with how Next.js handles server-side rendering for dynamically generated pages.

When a field in the Next-Router-State-Tree is manipulated, the server will still render the page according to the current routing state. However, the server may include the user DTO in the HTTP response because it is required to render the profile page correctly. This does not indicate a security flaw, but rather reflects how server components and dynamic routing work in Next.js.

Sending the user DTO to the client in this case is expected for proper rendering and hydration. It is not intended as a mechanism to reset the client, but simply to provide the necessary data for the server-rendered page to match the intended route and component state.

If you want to mitigate exposure of sensitive information, you can consider:

1- Filtering or sanitizing fields in the DTO before sending it to the client.

2- Implementing server-side checks to ensure only authorized data is returned per request.

3- Avoid relying on Next-Router-State-Tree as a client-side validation mechanism.

Overall, this behavior is typical and necessary for Next.js server-side rendering of dynamic pages. Focus on server-side safeguards for sensitive data, rather than trying to prevent the inclusion of the DTO entirely.

[zeaiter-squared]

Thank you for the assistance @Umuts-Codes.