Fastify middie patches child-scope middleware auth bypass

TL;DR — A Critical @fastify/middie middleware inheritance bug can silently drop parent-scope auth middleware from child plugins, allowing unauthenticated access to routes that teams assume are protected.

What happened

@fastify/middie is a middleware engine for the Fastify Node.js framework, commonly used to run Express-style middleware in Fastify applications.

CVE-2026-6270 describes a middleware authentication/authorization bypass where @fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. In affected setups, teams may register an authentication middleware in a parent scope and then register child plugins; the child scope can fail to inherit that parent middleware, letting unauthenticated requests reach routes defined in the child plugin scope.

This is an appsec-relevant failure mode because it produces a false sense of protection: the application can look “correct” in code review (auth middleware present) while effectively being bypassed due to scoping/inheritance behavior. This also continues a broader pattern of middleware/path/normalization edge-cases causing real-world auth bypasses in popular web stacks.

Who is impacted

• Fastify applications using @fastify/middie where authentication middleware is registered in a parent scope and routes are defined under child plugin scopes.
• Projects using @fastify/middie versions < 9.3.2.

Severity: CVSS v3.1 9.1 (Critical) (network, no privileges, no user interaction).

What to do now

• Follow vendor remediation guidance and apply the latest patched release available at the time of writing.

  "Upgrade to @fastify/middie 9.3.2 to fix this issue."

• Treat this as a potential auth bypass exposure: inventory services using @fastify/middie, and identify where parent-scope middleware is expected to protect child plugin routes.
• If you suspect exposure in an internet-facing service, review access logs for requests reaching child-scope routes that should have been gated by auth.
• Note the CVE record’s guidance on mitigations.

  "There are no workarounds."