CVE-2026-18504 PUBLISHED

fastify vulnerable to schema validation bypass via root primitive coercion mismatch

Assigner: openjs
Reserved: 31.07.2026 Published: 18.08.2026 Updated: 18.08.2026

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.

Metrics

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
CVSS Score: 5.4

Product Status

Vendor fastify
Product fastify
Versions Default: unaffected
  • affected from 0 to 5.12.1 (excl.)
  • Version 5.12.1 is unaffected

Credits

  • velgusgus599 reporter
  • mcollina remediation developer
  • UlisesGascon remediation reviewer

References

Problem Types

  • CWE-20: Improper Input Validation CWE