CVE-2026-6270: Authentication Bypass in @fastify/middie

The core of the issue lies in how @fastify/middie handles middleware path propagation to child plugin scopes. When a child plugin is registered with a prefix that overlaps with a parent-scoped middleware path, the middleware path is incorrectly modified. This silent modification prevents the middleware from matching incoming requests, effectively bypassing all security measures implemented through middleware. This includes authentication, authorization, rate limiting, and other critical security controls. The impact is particularly severe for applications relying on middleware for security enforcement, as attackers can bypass these protections entirely, gaining unauthorized access to resources and functionality within the affected child plugin scopes. Nested plugins (grandchild scopes) are also vulnerable.

1. Reserved2026-04-14
2. Published2026-04-16
3. EPSS updated2026-04-24

The primary mitigation is to upgrade to @fastify/middie version 9.3.2 or later, which contains the fix for this vulnerability. If upgrading is not immediately feasible, consider temporarily disabling or modifying affected child plugins to reduce the attack surface. Review your application's middleware configuration to identify potential overlaps between parent and child plugin scopes. Implement stricter input validation and output encoding to minimize the impact of potential exploits. Consider using a Web Application Firewall (WAF) to filter malicious requests targeting vulnerable endpoints, although this is not a substitute for patching.