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A GitHub-reviewed security advisory, not necessarily under active exploitation the way our CISA KEV advisories are, but a real, disclosed vulnerability in an open-source dependency.
Affected packages
Vulnerable: >= 1.143.12, < 1.169.39
Patched in 1.169.39
Vulnerable: >= 1.143.12, < 1.168.60
Patched in 1.168.60
Vulnerable: >= 1.143.12, < 1.168.57
Patched in 1.168.57
Vulnerable: >= 1.143.12, < 1.168.56
Patched in 1.168.56
Details
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.
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.
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/[email protected], @tanstack/[email protected], @tanstack/[email protected], and @tanstack/[email protected].
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.
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.
Lovable helped us discover and report this issue. We thank them for their help.
EPSS, exploit probability
Low0.52%
estimated chance of real-world exploitation in the next 30 days, higher than 42.6% of every CVE FIRST.org scores
Refreshed 10/16/2026, via FIRST.org's EPSS model, not CVSS, this measures likelihood of exploitation, not how severe it would be.
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A secure code review or supply chain review finds this before an advisory does.