Vulnerability · Published Oct 20, 2026 · Assigned by openjs
CVE-2026-112712@fastify/middie vulnerable to path-scoped middleware bypass via scope prefix mismatch
Critical 9.1
In plain English
A critical-severity security flaw in @fastify/middie from @fastify/middie. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).
Official 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 (as reported)
| Vendor | Product | Versions |
|---|---|---|
| @fastify/middie | @fastify/middie | 9.0.0 – before 9.3.6 |
Weakness: CWE-436 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N