---
id: CVE-2026-112712
title: >-
  @fastify/middie vulnerable to path-scoped middleware bypass via scope prefix
  mismatch
summary: >-
  @fastify/middie is the Express-style middleware engine for Fastify. In
  versions from 9.0.0 through 9.3.5, the use() function joined a path-scoped
  middleware's mount path to the plugin scope prefix in a way that could diverge
  from the pat…
severity: critical
cvss: 9.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N'
cvssSource: cna
cwe:
  - CWE-436
vendor: '@fastify/middie'
product: '@fastify/middie'
affected:
  - '@fastify/middie >= 9.0.0 < 9.3.6'
published: '2026-10-20'
updated: '2026-10-20'
sourceUpdated: '2026-10-20T20:43:48.681Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-112712'
references:
  - url: 'https://github.com/fastify/middie/security/advisories/GHSA-5p92-8765-9fv8'
  - url: 'https://cna.openjsf.org/security-advisories.html'
tags:
  - cve.org
ingestedAt: '2026-10-20T21:19:21.262Z'
---

## Overview

@fastify/middie is the Express-style middleware engine for Fastify. In versions from 9.0.0 through 9.3.5, the use() function joined a path-scoped middleware's mount path to the plugin scope prefix in a way that could diverge from the path Fastify's router serves, most often when the scope prefix ended in a slash, and also for array mount paths and for mount paths written without a leading slash. When the two paths disagreed, the middleware was silently skipped while the route handler still ran, so path-scoped middleware such as authentication, authorization, or rate limiting could be bypassed by a single unauthenticated request. The issue is fixed in @fastify/middie 9.3.6, and users should upgrade to 9.3.6 or later. As a workaround, enforce authorization in a Fastify hook such as preHandler that runs after the router has resolved the request.

## Affected

- `@fastify/middie >= 9.0.0 < 9.3.6`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.