Fastify's Content-Type header tab character allows body validation bypass
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:
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://hackerone.com/reports/3464114
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | fastify | < 5.7.2 | 5.7.2 |
References
- https://github.com/fastify/fastify/security/advisories/GHSA-jx2c-rxcm-jvmq
- https://nvd.nist.gov/vuln/detail/CVE-2026-25223
- https://github.com/fastify/fastify/commit/32d7b6add39ddf082d92579a58bea7018c5ac821
- https://hackerone.com/reports/3464114
- https://fastify.dev/docs/latest/Reference/Validation-and-Serialization
- https://github.com/fastify/fastify
- https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/content-type-parser.js#L125
- https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/validation.js#L272
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N- Weakness (CWE)
CWE-436- Platform
- npm
- Package
fastify- Disclosed
- 02.02.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-jx2c-rxcm-jvmq - CC BY 4.0
Vulnerability history: fastify
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 16
- First → latest
- 07.2018 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 8
- Medium 7
- Low 1
Disclosures per year
- 2018: 1
- 2019: 0
- 2020: 1
- 2021: 0
- 2022: 2
- 2023: 0
- 2024: 0
- 2025: 1
- 2026: 11
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Fastify's Content-Type header tab character allows body validation bypass this entry
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