React Router allows a DoS via cache poisoning by forcing SPA mode

Summary

After some research, it turns out that it is possible to force an application to switch to SPA mode by adding a header to the request. If the application uses SSR and is forced to switch to SPA, this causes an error that completely corrupts the page. If a cache system is in place, this allows the response containing the error to be cached, resulting in a cache poisoning that strongly impacts the availability of the application.

Details

The vulnerable header is X-React-Router-SPA-Mode; adding it to a request sent to a page/endpoint using a loader throws an error. Here is the vulnerable code :

To use the header, React-router must be used in Framework mode, and for the attack to be possible the target page must use a loader.

Steps to reproduce

Versions used for our PoC:

• "@react-router/node": "^7.5.0",
• "@react-router/serve": "^7.5.0",
• "react": "^19.0.0"
• "react-dom": "^19.0.0"
• "react-router": "^7.5.0"

1. Install React-Router with its default configuration in Framework mode (https://reactrouter.com/start/framework/installation)
2. Add a simple page using a loader (example: routes/ssr)

3. Send a request to the endpoint using the loader (/ssr in our case) adding the following header:

X-React-Router-SPA-Mode: yes

Notice the difference between a request with and without the header;

Normal request

With the header

Impact

If a system cache is in place, it is possible to poison the response by completely altering its content (by an error message), strongly impacting its availability, making the latter impractical via a cache-poisoning attack.

Credits

• Rachid Allam (zhero;)
• Yasser Allam (inzo_)