---
id: CVE-2026-76169
title: >-
  fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent
  under one plugin prefix to the custom not-found handler of a different sibling
  plugin, and invoke it without the preHandler hook declared for that handler
summary: >-
  fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent
  under one plugin prefix to the custom not-found handler of a different sibling
  plugin, and invoke it without the preHandler hook declared for that handler.
  The in…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'
cwe:
  - CWE-288
  - CWE-289
vendor: fastify
product: fastify
affected:
  - 'fastify >= 4.0.0, < 5.12.2'
patched:
  - fastify 5.12.2
published: '2026-09-04'
updated: '2026-09-15'
sourceUpdated: '2026-09-15T20:07:46.700'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-76169'
references:
  - url: 'https://cna.openjsf.org/security-advisories.html'
    label: ce714d77-add3-4f53-aff5-83d477b104bb
  - url: 'https://github.com/fastify/fastify/security/advisories/GHSA-p68q-wchp-6fh7'
    label: ce714d77-add3-4f53-aff5-83d477b104bb
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-76169.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-76169'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2528456'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-76169'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-76169'
tags:
  - nvd
  - csaf
  - vex
  - red-hat
epss: 0.0053
epssPercentile: 0.42334
ingestedAt: '2026-09-08T19:08:49.592Z'
---

## Overview

fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. An unauthenticated attacker can therefore reach an authentication-protected private fallback through an unrelated public prefix and read its full response, bypassing the authentication hook and breaking prefix encapsulation. Users should upgrade to fastify 5.12.2 or later.

## Affected

- `fastify >= 4.0.0, < 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-2026-76169.json)