TanStack Router is a client-first, server-capable, fully type-safe router and full-stack framework for the web. From 1.143.12 until 1.169.39 for @tanstack/start-server-core, 1.168.60 for @tanstack/react-start, 1.168.57 for @tanstack/solid-start, and 1.168.56 for @tanstack/vue-start, an unauthenticated attacker can place non-public fields in a server-function request payload so that attacker-controlled middleware state reaches the error path and a response-shaped object is accepted as an HTTP response. A crafted server-function URL can therefore return attacker-controlled HTML from the application's origin; when a visitor opens the URL, the injected script can access same-origin data and issue requests with the visitor's authority. This issue is fixed in @tanstack/start-server-core 1.169.39, @tanstack/react-start 1.168.60, @tanstack/solid-start 1.168.57, and @tanstack/vue-start 1.168.56.