fastify vulnerable to request validation bypass via skipped boolean false schemas

Why it is Amber

• CVSS 7.5 high
• Remotely exploitable without logging in
• On your watchlist: npm

Affected packages

References

• https://github.com/advisories/GHSA-hwr6-493r-vm6h
• 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/releases/tag/v5.12.2
• https://github.com/advisories/GHSA-hwr6-493r-vm6h

Original description

### 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.