CVE-2026-42211: React Router's vendored turbo-stream v2 allows arbitrary constructor invocation via TYPE_ERROR deserialization leading to Unauth RCE

When using React Router v7 in Framework Mode, there exists a combination of steps that could potentially allow unauthorized RCE through external requests. This first requires the application code to have an existing prototype pollution vulnerability. This can be leveraged into a 2-step attack in which the second step can trigger unauthorized RCE on the remote server.

[!NOTE] This does not impact your React Router application if you are using Declarative Mode (<BrowserRouter>) or Data Mode (createBrowserRouter/<RouterProvider>).