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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-25223: 0.86% estimated exploitation activity in the next 30 days; percentile 58.62 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

2 Feb 2026, 22:23 UTC

Last modified by the source

4 Feb 2026, 18:07 UTC

Source last checked

26 Oct 2026, 12:42 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jx2c-rxcm-jvmq

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-jx2c-rxcm-jvmq

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# Patch Next advisory brief: CVE-2026-25223

Advisory: Fastify's Content-Type header tab character allows body validation bypass
IDs: CVE-2026-25223, GHSA-jx2c-rxcm-jvmq
Record: https://patchnext.com/advisories/cve-2026-25223 (revision 2, source checked 26 Oct 2026, 12:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fastify 5.7.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fastify: affected < 5.7.2; fixed in 5.7.2

## Suggested inspection (read-only)
- `npm ls fastify --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fastify.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-jx2c-rxcm-jvmq (OSV record GHSA-jx2c-rxcm-jvmq lists affected ranges: npm fastify >=0 <5.7.2 (fixed))
- Verified: https://github.com/advisories/GHSA-jx2c-rxcm-jvmq (GitHub-reviewed advisory GHSA-jx2c-rxcm-jvmq lists affected ranges: npm fastify >=0 <5.7.2)

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