fastify versions before 5.12.2 decide whether to compile a request schema based on JavaScript truthiness, but JSON Schema Draft 7 defines the boolean false as a valid schema that rejects every instance. When an application assigns false to a route's body, querystring, params, or headers schema to deny all input, fastify treats it as a missing schema, compiles no validator, and runs the route handler on any request. An unauthenticated remote client can therefore reach a handler that a valid deny-all schema was intended to make unreachable, a complete validation bypass that can lead to unauthorized state changes or execution of disabled operations. Users should upgrade to fastify 5.12.2 or later.

Affected: npm: fastify

### 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.

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