fastify vulnerable to request validation bypass via skipped boolean false schemas
Impact
Fastify decided whether to validate a request part by checking its schema for JavaScript truthiness. JSON Schema Draft 7 defines the boolean false as a valid schema that rejects every instance, but because false is falsy, a route that set body, querystring, params, or headers to false had that part left uncompiled: no validator was attached and the request reached the handler. An application that used false as a deny-all schema to make a route unreachable was therefore fully bypassed, and an unauthenticated remote client could reach the handler with any input. The same applied to the documented query alias for querystring. This is a complete bypass rather than a weak-schema issue, since false is the strongest JSON Schema assertion and must always fail.
Patches
Request-part schemas are now selected by an explicit presence check rather than truthiness, so a boolean false (or true) schema is compiled and enforced, including through the query alias. Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release.
Workarounds
If upgrading is not immediately possible, express a deny-all request schema with an always-failing object schema instead of the boolean false (for example { "not": {} }), or reject the request in an onRequest hook.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | fastify | < 5.12.2 | 5.12.2 |
References
- https://github.com/fastify/fastify/security/advisories/GHSA-hwr6-493r-vm6h
- https://nvd.nist.gov/vuln/detail/CVE-2026-84469
- https://github.com/fastify/fastify/commit/7de6e81697778f9f41bd327a3b1c5c9a0d9637b4
- https://cna.openjsf.org/security-advisories.html
- https://github.com/fastify/fastify
- https://github.com/fastify/fastify/releases/tag/v5.12.2
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N- Weakness (CWE)
CWE-20- Platform
- npm
- Package
fastify- Disclosed
- 30.09.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-hwr6-493r-vm6h - 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.
- 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
- 2018: 1
- 2019: 0
- 2020: 1
- 2021: 0
- 2022: 2
- 2023: 0
- 2024: 0
- 2025: 1
- 2026: 11
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fastify vulnerable to request validation bypass via skipped boolean false schemas this entry
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