---
id: CVE-2026-84428
title: >-
  fastify versions before 5.12.2 implement the case-insensitive nature of HTTP
  header names by lowercasing names in a route's header schema before compiling
  it, but the transformation is incomplete: it lowercases the properties keys
  and th…
summary: >-
  fastify versions before 5.12.2 implement the case-insensitive nature of HTTP
  header names by lowercasing names in a route's header schema before compiling
  it, but the transformation is incomplete: it lowercases the properties keys
  and th…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N'
cwe:
  - CWE-178
  - CWE-1289
vendor: fastify
product: fastify
affected:
  - fastify < 5.12.2
patched:
  - fastify 5.12.2
published: '2026-09-04'
updated: '2026-09-15'
sourceUpdated: '2026-09-15T19:54:32.420'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-84428'
references:
  - url: 'https://cna.openjsf.org/security-advisories.html'
    label: ce714d77-add3-4f53-aff5-83d477b104bb
  - url: 'https://github.com/fastify/fastify/security/advisories/GHSA-9q9j-q6p8-xq58'
    label: ce714d77-add3-4f53-aff5-83d477b104bb
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-84428.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-84428'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2528468'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-84428'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-84428'
tags:
  - nvd
  - csaf
  - vex
  - red-hat
epss: 0.00524
epssPercentile: 0.41964
ingestedAt: '2026-09-08T19:08:49.592Z'
---

## Overview

fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

## Affected

- `fastify < 5.12.2`

## Remediation

Upgrade past the affected range:

- `fastify 5.12.2`

## Vendor advisories

- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux AI (RHEL AI) 3, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift Dev Spaces · no fix planned: Red Hat Enterprise Linux AI (RHEL AI) 3, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift Dev Spaces · updated 2026-09-18 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-84428.json)