fastify vulnerable to schema validation bypass via root primitive coercion mismatch

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-18504: 0.28% estimated exploitation activity in the next 30 days; percentile 20.58 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

2 Sep 2026, 15:13 UTC

Last modified by the source

2 Sep 2026, 15:13 UTC

Source last checked

26 Oct 2026, 13:02 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-w2qp-rph6-63g4

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# Patch Next advisory brief: CVE-2026-18504

Advisory: fastify vulnerable to schema validation bypass via root primitive coercion mismatch
IDs: CVE-2026-18504, GHSA-w2qp-rph6-63g4
Record: https://patchnext.com/advisories/cve-2026-18504 (revision 1, source checked 26 Oct 2026, 13:02 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fastify 5.12.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fastify: affected < 5.12.1; fixed in 5.12.1

## Suggested inspection (read-only)
- `npm ls fastify --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fastify.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-w2qp-rph6-63g4 (GitHub-reviewed advisory GHSA-w2qp-rph6-63g4 lists affected ranges: npm fastify >=0 <5.12.1)

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