Vulnerability 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.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify | < 5.12.1 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlThird Party Advisory
- https://github.com/fastify/fastify/security/advisories/GHSA-w2qp-rph6-63g4MitigationVendor Advisory
FAQ
What is CVE-2026-18504?
CVE-2026-18504 is a vulnerability with a CVSS score of 5.4 (MEDIUM). 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. Wh...
How severe is CVE-2026-18504?
CVE-2026-18504 has been rated MEDIUM with a CVSS base score of 5.4/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-18504?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify.