Disclosure summary
### 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 unch
Source-specific records & product guidance
Sources retain their own attribution and scoring. Follow the original record to confirm affected versions, fixed releases, and configuration conditions.
GitHub Reviewed Security Advisories · GHSA-667r-xxjv-c9mm
Open original source · Updated Sep 30, 2026
fastify vulnerable to request body replacement via an async validation result collision
Source severity: HIGH / 0
| Ecosystem | Package | Affected range | First patched |
|---|---|---|---|
| npm | fastify | < 5.12.2 | 5.12.2 |
Original records & references
- NIST NVD record
- CVE Program record
- github.com — Reviewed advisory
PUBLISHED 2026-09-30T19:45:45-04:00
MODIFIED 2026-09-30T19:45:46-04:00
INGESTED 2026-10-06T11:45:02-04:00