Reference page — cumulative record through Sunday, October 4, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2026-18504

CVE-2026-18504MEDIUM

fastify vulnerable to schema validation bypass via root primitive coercion mismatch

  AV  AC  PR  UI  S  C  I  A   CVSS    EPSS   %ile   KEV
   N   L   N   N  U  N  L  N    5.3   .0031   21.6     —

AFFECTED
  Product  Versions     Fixed
  fastify  unspecified  5.12.1

TIMELINE
  Jul 31  Reserved by openjs
  Aug 18  Published (CNA: openjs)
  Sep 2   RESCORED — CVE-2026-18504 (fastify). CVSS 5.4 → 5.3 (NVD).

CWE-20 · CNA: openjs · CVSS v3.1 · 2 references · NVD status: Analyzed

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.

Lifecycle

Complete event history — 3 events, chronological
Date	Event	Detail
July 31, 2026	Reserved	Reserved by openjs
August 18, 2026	Published	Published (CNA: openjs)
September 2, 2026	RESCORED	RESCORED — CVE-2026-18504 (fastify). CVSS 5.4 → 5.3 (NVD).

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
fastify	fastify	—	—	5.12.1

Weaknesses

CWE-20

References (2)

https://cna.openjsf.org/security-advisories.html [Third Party Advisory]
https://github.com/fastify/fastify/security/advisories/GHSA-w2qp-rph6-63g4 [Mitigation, Vendor Advisory]

Authoritative record: CVE-2026-18504 at cve.org

Vendors: fastify

Weaknesses: CWE-20

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2026-18504 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Sunday, October 4, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.