CVE-2026-6270: Fastify Fastify/middie

Critical severity, CVSS 9.1. EPSS: 0.6% chance of exploitation in the next 30 days.

@fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. When a Fastify application registers authentication middleware in a parent scope and then registers child plugins with @fastify/middie, the child scope does not inherit the parent middleware. This allows unauthenticated requests to reach routes defined in child plugin scopes, bypassing authentication and authorization checks. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds.

Affected products

• Fastify Fastify/middie: before 9.3.2 (fixed in 9.3.2)

Published 2026-04-16. Last modified 2026-06-17.