# TanStack Start Server Core type confusion in request deserialization

Source: https://junglewise.ai/threats/tanstack-start-server-core-type-confusion-in-request-deserialization-53e6cf74  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-14. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

Severity: medium · CVSS 4 · Published 2026-05-14

TanStack Start is a full-stack web framework that handles server-side function calls from web applications. A type-confusion flaw in the deserialization layer allows an attacker to craft a malicious request to one server function that inadvertently triggers execution of a different server function. For correctly secured applications (those with proper authentication and input validation), the practical impact is limited, but organizations with legacy or improperly configured server functions risk unintended side effects and audit trail confusion.

Technologies: [@tanstack/start-server-core (npm)](https://junglewise.ai/threats/technologies/tanstack-start-server-core). Vendors: [npm](https://junglewise.ai/threats/vendors/npm), [TanStack](https://junglewise.ai/threats/vendors/tanstack).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Medium severity, with a CVSS score of 4, published 14 May 2026.

**Which products and versions are affected?** @tanstack/start-server-core (npm) SEMVER:introduced=0,fixed=1.167.30; TanStack Start Server Core <1.167.30.

**Is there a fix for This vulnerability?** Yes. @tanstack/start-server-core (npm): upgrade to 1.167.30 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network.

**Am I affected?** Check whether you run @tanstack/start-server-core (npm) or TanStack Start Server Core at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- @tanstack/start-server-core (npm): upgrade to 1.167.30 or later

  ```sh
  npm install @tanstack/start-server-core@1.167.30
  ```


## Technical details

A type-confusion bug in seroval ≤ 1.5.2 (a deserialization library used by TanStack Start) allows a crafted JSON request body to one server function to invoke a different client-referenced server function as a side effect of payload deserialization. The vulnerability is rooted in how seroval handles adapter payloads that can be confused with internal node types. The attack vector is network-based and requires no authentication or user interaction, but exploitation is only meaningful if the target server function lacks both authentication/authorization middleware and input validation. Notably, server-only functions marked as `isClientReferenced: false` cannot be reached through this mechanism. Patches are available in @tanstack/start-server-core ≥ 1.167.30, which upgrades seroval to ≥ 1.5.3 and adds defense-in-depth serialization protection.

## Affected products

- TanStack Start Server Core <1.167.30

## Timeline

- 2026-05-14: disclosed: Advisory GHSA-9m65-766c-r333 published
- 2026-05-14: patched: Patch available in version 1.167.30 and equivalent dated releases

## References

- [https://github.com/TanStack/router/security/advisories/GHSA-9m65-766c-r333](https://github.com/TanStack/router/security/advisories/GHSA-9m65-766c-r333)
- [https://github.com/TanStack/router](https://github.com/TanStack/router)

## Related threats

- [TanStack Start reflected XSS in server-function responses](https://junglewise.ai/threats/cve-2026-102989-tanstack-router-unauthenticated-reflected-xss-in-tanstack-start)
- [TanStack npm packages supply chain compromise via GitHub Actions OIDC hijacking](https://junglewise.ai/threats/cve-2026-45321-tanstack-npm-packages-supply-chain-compromise-via-github-actions)