CVE-2026-6270: fastify fastify\/middie Interpretation Conflict

Vulnerability summary

CVE-2026-6270 is a critical (CVSS 9.1) Interpretation Conflict vulnerability in fastify fastify\/middie. EPSS estimates a 0.61% chance of exploitation in the next 30 days, higher than 48.1% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 16-04-2026
   PublishedVulnerability first disclosed.
2. 16-04-2026
   Last modifiedVulnerability information updated.

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.

• CWE-436Interpretation ConflictProduct A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.