Summary
TanStack Start - Server Core: Inbound server-function request deserialization could invoke a sibling client-referenced server function
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/, and the target function's full middleware chain, including any user-supplied authentication, authorization, and inputValidator, runs as it would on a direct call.
Workarounds
If you cannot upgrade immediately, ensure every createServerFn(...) exposed to the client has both an .inputValidator(...) and authentication/authorization middleware via .middleware([...]). This is recommended regardless of this advisory.
Credits
- Mufeed VH of Winfunc Research
- Upstream fix coordinated with Seroval maintainers https://github.com/lxsmnsyc/seroval
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 impact on correctly-written applications is nil. The residual concerns are:
A request to function A could cause function B to also execute, which may surprise observability/audit logging that keys off the request URL.
Request-level middleware (as opposed to per-function middleware) does not re-run for the inner invocation.
Server-only functions (isClientReferenced: false) cannot be reached through this mechanism.
Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Upgrade to @tanstack/start-server-core ≥ 1.167.30 (or the equivalent dated release of @tanstack/react-start / @tanstack/solid-start). The fix bumps seroval to ≥ 1.5.3 and adds defense-in-depth to the serialization adapter plugin shape so adapter payloads cannot be confused with internal seroval node types.
Frequently Asked Questions
- What is GHSA-9M65-766C-R333? GHSA-9M65-766C-R333 is a medium-severity insecure deserialization vulnerability in @tanstack/start-server-core (npm), affecting versions < 1.167.30. It is fixed in 1.167.30. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
- Which versions of @tanstack/start-server-core are affected by GHSA-9M65-766C-R333? @tanstack/start-server-core (npm) versions < 1.167.30 is affected.
- Is there a fix for GHSA-9M65-766C-R333? Yes. GHSA-9M65-766C-R333 is fixed in 1.167.30. Upgrade to this version or later.
- Is GHSA-9M65-766C-R333 exploitable, and should I be worried? Whether GHSA-9M65-766C-R333 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-9M65-766C-R333 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-9M65-766C-R333? Upgrade
@tanstack/start-server-coreto 1.167.30 or later.