CVE-2026-25223

Fastify is a fast and low overhead web framework, for Node.js. Prior to version 5.7.2, a validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (\t) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type. This issue has been patched in version 5.7.2.

Fastify is a fast and low overhead web framework, for Node.js. Prior to version 5.7.2, a validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (\t) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type. This issue has been patched in version 5.7.2.

### Impact A validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (`\t`) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type. For example, a request with `Content-Type: application/json\ta` will bypass JSON schema validation but still be parsed as JSON. This vulnerability affects all Fastify users who rely on Content-Type-based body validation schemas to enforce data integrity or security constraints. The concrete impact depends on the handler implementation and the level of trust placed in the validated request body, but at the library level, this allows complete bypass of body validation for any handler using Content-Type-discriminated schemas. This issue is a regression or missed edge case from the fix for a previously reported vulnerability. ### Patches This vulnerability has been patched in **Fastify v5.7.2**. All users should upgrade to this version or later immediately. ### Workarounds If upgrading is not immediately possible, user can implement a custom `onRequest` hook to reject requests containing tab characters in the Content-Type header: ```javascript fastify.addHook('onRequest', async (request, reply) => { const contentType = request.headers['content-type'] if (contentType && contentType.includes('\t')) { reply.code(400).send({ error: 'Invalid Content-Type header' }) } }) ``` ### Resources - https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/validation.js#L272 - https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/content-type-parser.js#L125 - [Fastify Validation and Serialization Documentation](https://fastify.dev/docs/latest/Reference/Validation-and-Serialization/) - https://hackerone.com/reports/3464114

### Impact A validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (`\t`) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type. For example, a request with `Content-Type: application/json\ta` will bypass JSON schema validation but still be parsed as JSON. This vulnerability affects all Fastify users who rely on Content-Type-based body validation schemas to enforce data integrity or security constraints. The concrete impact depends on the handler implementation and the level of trust placed in the validated request body, but at the library level, this allows complete bypass of body validation for any handler using Content-Type-discriminated schemas. This issue is a regression or missed edge case from the fix for a previously reported vulnerability. ### Patches This vulnerability has been patched in **Fastify v5.7.2**. All users should upgrade to this version or later immediately. ### Workarounds If upgrading is not immediately possible, user can implement a custom `onRequest` hook to reject requests containing tab characters in the Content-Type header: ```javascript fastify.addHook('onRequest', async (request, reply) => { const contentType = request.headers['content-type'] if (contentType && contentType.includes('\t')) { reply.code(400).send({ error: 'Invalid Content-Type header' }) } }) ``` ### Resources - https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/validation.js#L272 - https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/content-type-parser.js#L125 - [Fastify Validation and Serialization Documentation](https://fastify.dev/docs/latest/Reference/Validation-and-Serialization/) - https://hackerone.com/reports/3464114

Fastify es un framework web rápido y de baja sobrecarga, para Node.js. Antes de la versión 5.7.2, existe una vulnerabilidad de omisión de validación en Fastify donde los esquemas de validación del cuerpo de la solicitud especificados por Content-Type pueden ser completamente eludidos. Al añadir un carácter de tabulación (\t) seguido de contenido arbitrario al encabezado Content-Type, los atacantes pueden omitir la validación del cuerpo mientras el servidor sigue procesando el cuerpo como el tipo de contenido original. Este problema ha sido parcheado en la versión 5.7.2.