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Medium

fastify vulnerable to schema validation bypass via root primitive coercion mismatch

  • Disclosed: 02.09.2026
  • Updated: 11.10.2026 10:22

Impact

fastify before 5.12.1, when a route uses a root-level primitive body schema (for example an integer with a minimum and maximum) and the default type coercion, validates the coerced value but exposes the original, uncoerced value to the route handler. For example, a JSON body "10" is coerced to the number 10 and passes an integer 1 to 10 schema, but request.body stays the string "10". An application that trusts the validated type is handed a value that did not satisfy the schema, which can bypass limits the application enforces on that typed value. Object and array body schemas are not affected, they coerce their members in place.

Patches

Upgrade to fastify 5.12.1.

Workarounds

Until you can upgrade, avoid relying on the validated type of a root primitive body. Wrap the value in an object schema (object properties are coerced in place), for example accept { "value": 10 } and read request.body.value, or re-check the type in the handler.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
npm package fastify < 5.12.1 5.12.1

References

CVSS score 5.4/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
Weakness (CWE)
CWE-20
Platform
npm
Package
fastify
Disclosed
02.09.2026
Updated
11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-w2qp-rph6-63g4 - CC BY 4.0

Vulnerability history: fastify

All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (16) in the archive
Total entries
16
First → latest
07.2018 09.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 8
  • Medium 7
  • Low 1

Disclosures per year

Severity:
  1. fastify vulnerable to request body replacement via an async validation result collision
    High · 8.1 CVE-2026-84504 < 5.12.2 5.12.2
  2. fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
    High · 7.5 CVE-2026-76169 >= 4.0.0, < 5.12.2 5.12.2
  3. fastify vulnerable to request validation bypass via skipped boolean false schemas
    High · 7.5 CVE-2026-84469 < 5.12.2 5.12.2
  4. fastify vulnerable to header validation bypass via incomplete schema case normalization
    High · 7.5 CVE-2026-84428 < 5.12.2 5.12.2
  5. fastify vulnerable to schema validation bypass via root primitive coercion mismatch this entry
    Medium · 5.4 CVE-2026-18504 < 5.12.1 5.12.1
  6. Fastify's Content-Type header tab character allows body validation bypass
    High · 7.5 CVE-2026-25223 < 5.7.2 5.7.2
  7. Fastify vulnerable to invalid content-type parsing, which could lead to validation bypass
    High · 7.5 CVE-2025-32442 >= 5.0.0, < 5.3.2, >= 4.29.0, < 4.29.1 5.3.2, 4.29.1
  8. fastify vulnerable to denial of service via malicious Content-Type
    High · 7.5 CVE-2022-39288 >= 4.0.0, < 4.8.1 4.8.1
  9. Denial of Service vulnerability with large JSON payloads in fastify
    High · 7.5 CVE-2018-3711 < 0.38.0 0.38.0

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