CVE-2026-25223

Description

### 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

How to fix CVE-2026-25223

To remediate CVE-2026-25223, upgrade the affected package to a fixed version below.

• —upgrade to 5.7.2 or later

Is CVE-2026-25223 being exploited?

Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.