@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 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.

Evidence

Reported to the maintainer with a proof of concept that reproduced against the affected release.

Remediation

Upgrade @fastify/middie to 9.3.6 or later.

References

• Referencehttps://github.com/fastify/middie/security/advisories/GHSA-5p92-8765-9fv8
• Referencehttps://cna.openjsf.org/security-advisories.html
• CVE recordhttps://www.cve.org/CVERecord?id=CVE-2026-112712