CVE-2026-18504
fastify vulnerable to schema validation bypass via root primitive coercion mismatch
- Published
- Aug 18, 2026
- Updated
- Aug 19, 2026
- Assigning CNA
- openjs
- Evidence observed
- Aug 19, 2026
Primary CVSS
nvd · CVSS 3.1
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:NLow · next 30 days
- Percentile
- 23.9%
- Model date
- Sep 21, 2026
EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.
Summary
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.
Sources
1Educational repository for documenting and testing CVE proof-of-concept exploits inside isolated labs, virtual machines, and authorized penetration testing environments.
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