Fastify Middleware Flaw Exposes Apps to Auth Bypass

The National Vulnerability Database is flagging a critical vulnerability affecting the @fastify/middie package, a middleware handler for the popular Fastify web framework. Versions prior to 9.3.2 suffer from a flaw where middleware registered in a parent scope isn’t properly applied to child plugin instances. This oversight means that if you’re setting up authentication in a main Fastify application and then nesting child plugins handled by @fastify/middie, those child routes might not inherit the parent’s authentication checks.

This vulnerability, tracked as CVE-2026-6270, has a CVSS score of 9.1, landing it squarely in the ‘Critical’ severity bracket. The National Vulnerability Database notes that it allows unauthenticated requests to bypass security controls and reach routes within these child plugin scopes. Essentially, attackers could waltz right in if they hit the right endpoint, sidestepping authentication and authorization mechanisms entirely. The issue is rooted in CWE-436, which deals with interpretational differences in computer code.

According to the National Vulnerability Database, there are no workarounds for this bug. The only remedy is to upgrade to @fastify/middie version 9.3.2 or later. If your Fastify applications utilize nested plugins and rely on parent-scope middleware for security, this is a must-fix. The affected products beyond the @fastify/middie package itself aren’t specified, but the impact could be widespread across any Fastify deployment using this pattern.