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CRITICAL severity

TanStack Router: Unauthenticated reflected XSS in TanStack Start server-function responsesGHSA-qx66-fv34-fjm8

CRITICALFix: TanStack/router#8573

GHSA-qx66-fv34-fjm8 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.

Also known asCVE-2026-102989
Published
Updated
Affected
4 pkgs
Patched
4 / 4
Exploits
None indexed
Exploitation data as of Oct 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

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 GHSA-qx66-fv34-fjm8.

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs43th percentile — riskier than 43% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-qx66-fv34-fjm8 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

4 pkgs affected

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.

19other npm packages depend on this — each one inherits the vulnerability until it's patched upstream
@tanstack/start-server-corenpm
7.2Mdownloads / week
@tanstack/react-startnpm
7.2Mdownloads / week
@tanstack/solid-startnpm
46Kdownloads / week

Description

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

4 total 4 fixed
EcosystemPackageVulnerable rangeFix
📦npm@tanstack/start-server-core≥ 1.143.12&&< 1.169.391.169.39npm install @tanstack/start-server-core@1.169.39
📦npm@tanstack/react-start≥ 1.143.12&&< 1.168.601.168.60npm install @tanstack/react-start@1.168.60
📦npm@tanstack/solid-start≥ 1.143.12&&< 1.168.571.168.57npm install @tanstack/solid-start@1.168.57
📦npm@tanstack/vue-start≥ 1.143.12&&< 1.168.561.168.56npm install @tanstack/vue-start@1.168.56

Detection & mitigation playbook

Open-source dependency

  1. Detect

    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.

  2. 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 GHSA-qx66-fv34-fjm8 is resolved across your whole dependency graph.

  3. 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

## 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 ca
O3 Security · Impact-Aware SCA

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