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Critical · 9.1 CVE-2026-112712 EUVD-2026-105361

@fastify/middie vulnerable to path-scoped middleware bypass via scope prefix mismatch

Published Oct 20, 2026 · Updated Oct 30, 2026 · Assigned by openjs

Description

@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 products

VendorProductAffectedFixed
@fastify/middie @fastify/middie 9.0.0 – < 9.3.6 9.3.6

References