Fastify's Content-Type header tab character allows body validation bypass

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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.

Source published

3 Feb 2026 21:21:40 UTC

Source modified

16 Sep 2026 12:04:20 UTC

Saved here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

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.

Source published

Feb 3, 2026, 9:21:40 PM (timezone not stated)

Source modified

Sep 16, 2026, 12:04:20 PM (timezone not stated)

Saved here

13 Nov 2026 11:01:55 UTC

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ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

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.

Source published

3 Feb 2026 22:16:31 UTC

Source modified

15 Jul 2026 02:18:52 UTC

Saved here

25 Oct 2026 06:17:16 UTC

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CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS 3.1 · 0b0ca135-0b70-47e7-9f44-1890c2a1c46c · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

### 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

Source published

2 Feb 2026 22:23:29 UTC

Source modified

4 Feb 2026 18:07:32 UTC

Saved here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Publisher rating · HIGH