CVE-2026-84504 is a high-severity (CVSS 8.1) Improper Input Validation vulnerability in fastify. A fix is available for fastify — see the affected versions and patch details below.
fastify vulnerable to request body replacement via an async validation result collision
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-2026-84504.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-84504 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,415 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
fastifyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Fastify runs a route's validator and, for a result shaped like { value, error }, unwraps it: an error becomes a validation failure and value replaces the request part. This convention is intended for synchronous custom compilers (for example Joi). A JSON Schema $async validator, however, resolves with the validated data itself, so Fastify applied the same unwrapping to it. If a request part validated by an $async schema contains a value property, Fastify replaced the whole request part with that nested value before the handler ran, so a value or error property in the payload was attacker-controlled. An application that dispatches operations from the validated request body could then act on data that never satisfied the route schema, leading to unauthorized state changes or disclosure. Reaching the vulnerable path requires the route to use an $async request schema.
Patches
Fastify no longer treats an asynchronous validation result as a { value, error } wrapper: an async validator's resolved value is used only to determine pass or fail, and it can no longer replace the request part or inject an error. The synchronous custom-compiler contract is unchanged. Patched in fastify 5.12.2 and 6.0.0.
Workarounds
If upgrading is not immediately possible, avoid $async request schemas, or perform the security-sensitive check in an onRequest or preHandler hook rather than relying on the schema-validated request part. Custom async validator compilers should signal failure by throwing (rejecting) rather than returning an { error } object.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fastify | all versions | 5.12.2npm install fastify@5.12.2 |
Affected Products
fastifyfastifyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastify to 5.12.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-84504 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Important flaw in fastify allows an authenticated low-privilege user to bypass schema validation. By injecting a 'value' property into the request body, an attacker can trigger unauthorized state changes and data disclosure, impacting the integrity and confidentiality of data in affected Red Hat OpenShift AI and…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-84504 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-84504 in your dependencies?
Find it across npm, including transitive dependencies.