TanStack Router: Unauthenticated reflected XSS in TanStack Start server-function responses

## Impact An unauthenticated attacker can craft a URL to a server function in an affected TanStack Start app. If someone opens the link, the app can return attacker-controlled HTML from its own origin and run the attacker's JavaScript there. The script can access same-origin data available to that visitor and make requests with their authority. ## Cause The server-function transport can pass request payload fields into internal middleware state instead of limiting them to public input fields. A malformed payload can leave an attacker-controlled result in the error path. Response handling can then treat a response-shaped object as an HTTP response. ## Affected versions Published versions of `@tanstack/start-server-core`, `@tanstack/react-start`, `@tanstack/solid-start`, and `@tanstack/vue-start` from `1.143.12` up to, but not including, their respective patched versions are affected. The patched versions are `@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`. ## Fix The fix limits client-supplied server-function input to public fields and validates responses at the server boundary. Upgrade to the corresponding patched package version. ## Temporary mitigations For the configured server-function path, usually `/_serverFn/*`, an edge or WAF rule can consider blocking requests without the literal `x-tsr-serverFn: true` header and browser document navigations such as `Sec-Fetch-Mode: navigate` or `Accept: text/html`. A restrictive Content Security Policy on server-function responses can provide another browser-side restriction. These are mitigation suggestions, not a complete fix. The header is client-controlled and is not authentication. Test rules against legitimate GET and POST server functions and use a matching version-specific backport where possible. ## Credits Lovable helped us discover and report this issue. We thank them for their help. Source Code Location: https://github.com/TanStack/router Affected Packages: - npm:@tanstack/start-server-core, affected >= 1.143.12, < 1.169.39, patched in 1.169.39 - npm:@tanstack/react-start, affected >= 1.143.12, < 1.168.60, patched in 1.168.60 - npm:@tanstack/solid-start, affected >= 1.143.12, < 1.168.57, patched in 1.168.57 - npm:@tanstack/vue-start, affected >= 1.143.12, < 1.168.56, patched in 1.168.56 CWEs: - CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') CVSS: - Primary: score 9.3, CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N - CVSS_V3: score 9.3, CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N References: - https://github.com/TanStack/router/security/advisories/GHSA-qx66-fv34-fjm8 - https://nvd.nist.gov/vuln/detail/CVE-2026-102989 - https://github.com/TanStack/router/pull/8573 - https://github.com/TanStack/router/commit/d521abd71c69f1ff010bb3def4d417cb5fa058cb - https://github.com/TanStack/router/releases/tag/release-2026-09-30-1747 - https://github.com/advisories/GHSA-qx66-fv34-fjm8