fastify vulnerable to schema validation bypass via root primitive coercion mismatch

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-18504
StatePUBLISHED
Assigneropenjs
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-18 21:16:34 UTC
Updated2026-09-02 14:30:22 UTC
Descriptionfastify 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.

Risk And Classification

Primary CVSS: v3.1 5.3 MEDIUM from [email protected]

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

EPSS: 0.003120000 probability, percentile 0.236090000 (date 2026-09-02)

Problem Types: CWE-20 | CWE-20 CWE-20: Improper Input Validation

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
3.1ce714d77-add3-4f53-aff5-83d477b104bbSecondary5.4MEDIUMCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
3.1CNACVSS5.4MEDIUMCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Fastify Fastify All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Fastify Fastify affected 5.12.1 semver Not specified
CNA Fastify Fastify unaffected 5.12.1 semver Not specified

References

ReferenceSourceLinkTags
cna.openjsf.org/security-advisories.html ce714d77-add3-4f53-aff5-83d477b104bb cna.openjsf.org Third Party Advisory
github.com/fastify/fastify/security/advisories/GHSA-w2qp-rph6-63g4 ce714d77-add3-4f53-aff5-83d477b104bb github.com Mitigation, Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: velgusgus599 (en)

CNA: mcollina (en)

CNA: UlisesGascon (en)

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