React Router is a router for React. In versions on the 7.0 branch prior to version 7.5.2, it's possible to modify pre-rendered data by adding a header to the request. This allows to completely spoof its contents and modify all the values of the data object passed to the HTML. This issue has been patched in version 7.5.2.
https://github.com/remix-run/react-router/security/advisories/GHSA-cpj6-fhp6-mr6j
https://github.com/remix-run/react-router/commit/c84302972a152d851cf5dd859ff332b354b70111