CVE-2026-102989 is a critical-severity (CVSS 9.3) Cross-site Scripting (XSS) vulnerability in @tanstack/start-server-core. A fix is available for @tanstack/start-server-core — see the affected versions and patch details below.
TanStack Router: Unauthenticated reflected XSS in TanStack Start server-function responses
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-102989.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-102989 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@tanstack/start-server-corenpm@tanstack/react-startnpm@tanstack/solid-startnpmDescription
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/[email protected], @tanstack/[email protected], @tanstack/[email protected], and @tanstack/[email protected].
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @tanstack/start-server-core | ≥ 1.143.12&&< 1.169.39 | 1.169.39npm install @tanstack/start-server-core@1.169.39 |
| 📦npm | @tanstack/react-start | ≥ 1.143.12&&< 1.168.60 | 1.168.60npm install @tanstack/react-start@1.168.60 |
| 📦npm | @tanstack/solid-start | ≥ 1.143.12&&< 1.168.57 | 1.168.57npm install @tanstack/solid-start@1.168.57 |
| 📦npm | @tanstack/vue-start | ≥ 1.143.12&&< 1.168.56 | 1.168.56npm install @tanstack/vue-start@1.168.56 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @tanstack/start-server-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @tanstack/start-server-core to 1.169.39 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-102989 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2026-102989 in your dependencies?
Find it across npm, including transitive dependencies.