TanStack Start - Server Core: Inbound server-function request deserialization could invoke a sibling client-referenced server function

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary A type-confusion bug in seroval ≤ 1.5.2 (upstream advisory) allowed a crafted JSON body sent to one TanStack Start server function to trigger invocation of a different client-referenced server function as a side effect of deserializing the request payload. This is not an authentication bypass and not remote code execution. The mechanism only invokes server functions that the same client could already reach directly via /serverFn/<id, and the target function's full middleware chain — including any user-supplied authentication, authorization, and inputValidator — runs as it would on a direct call. Impact To be exploitable in any meaningful sense, an application would need to expose a client-referenced server function that: - [ ] Performs a privileged side effect, and - [ ] Has no authentication/authorization middleware, and - [ ] Has no input validation A function meeting all three is already directly callable by any unauthenticated client at its own endpoint, so the practical…

Published by the source

14 May 2026, 16:37 UTC

Last modified by the source

14 May 2026, 16:57 UTC

Source last checked

26 Oct 2026, 12:52 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-9m65-766c-r333

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-9m65-766c-r333

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# Patch Next advisory brief: GHSA-9m65-766c-r333

Advisory: TanStack Start - Server Core: Inbound server-function request deserialization could invoke a sibling client-referenced server function
IDs: GHSA-9m65-766c-r333
Record: https://patchnext.com/advisories/ghsa-9m65-766c-r333 (revision 3, source checked 26 Oct 2026, 12:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @tanstack/start-server-core 1.167.30. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @tanstack/start-server-core: affected < 1.167.30; fixed in 1.167.30

## Suggested inspection (read-only)
- `npm ls @tanstack/start-server-core --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @tanstack/start-server-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-9m65-766c-r333 (OSV record GHSA-9m65-766c-r333 lists affected ranges: npm @tanstack/start-server-core >=0 <1.167.30 (fixed))
- Verified: https://github.com/advisories/GHSA-9m65-766c-r333 (GitHub-reviewed advisory GHSA-9m65-766c-r333 lists affected ranges: npm @tanstack/start-server-core >=0 <1.167.30)

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