EUVD-2026-363894

fkie_cve-2026-84504

fastify versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value result protocol used by custom validator compilers. If a request that passes its route schema contains a property named value at the root, fastify replaces the entire request body with that property's value before the handler runs, so the handler receives a different object than the one that satisfied the schema. An authenticated low-privilege caller can use this to make nested data replace the validated body and trigger an operation the route schema did not authorize, leading to unauthorized state changes and data disclosure. Users should upgrade to fastify 5.12.2 or later.

GHSA-667r-xxjv-c9mm — fastify vulnerable to request body replacement via an async validation result collision

Affected: npm: fastify

### Impact

Fastify runs a route's validator and, for a result shaped like `{ value, error }`, unwraps it: an `error` becomes a validation failure and `value` replaces the request part. This convention is intended for synchronous custom compilers (for example Joi). A JSON Schema `$async` validator, however, resolves with the validated data itself, so Fastify applied the same unwrapping to it. If a request part validated by an `$async` schema contains a `value` property, Fastify replaced the whole request part with that nested value before the handler ran, so a `value` or `error` property in the payload was attacker-controlled. An application that dispatches operations from the validated request body could then act on data that never satisfied the route schema, leading to unauthorized state changes or disclosure. Reaching the vulnerable path requires the route to use an `$async` request schema.

### Patches

Fastify no longer treats an asynchronous validation result as a `{ value, error }` wrapper: an async validator's resolved value is used only to determine pass or fail, and it can no longer replace the request part or inject an error. The synchronous custom-compiler contract is unchanged. Patched in fastify `5.12.2` and `6.0.0`.

### Workarounds

If upgrading is not immediately possible, avoid `$async` request schemas, or perform the security-sensitive check in an `onRequest` or `preHandler` hook rather than relying on the schema-validated request part. Custom async valida…

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EUVD-2026-363894

fkie_cve-2026-84504

GHSA-667r-xxjv-c9mm — fastify vulnerable to request body replacement via an async validation result collision