CVE-2026-6270

Description

@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.

CVSS Metrics

• v3.1•CRITICAL•Score: 9.1CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Techniques & Countermeasures

• CWE-436•Interpretation Conflict

  Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.

Affected Systems

• @fastify/middie•@fastify/middie

  < 9.3.2

References (3)

• https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c
• https://github.com/fastify/middie/security/advisories/GHSA-72c6-fx6q-fr5w
• https://cna.openjsf.org/security-advisories.html