CVE-2026-112712: @fastify/middie Interpretation Conflict

Vulnerability summary

CVE-2026-112712 is a critical (CVSS 9.1) Interpretation Conflict vulnerability in @fastify/middie. EPSS estimates a 0.31% chance of exploitation in the next 30 days, higher than 23% of scored vulnerabilities.

Timeline

1. 20-10-2026
   PublishedVulnerability first disclosed.

Description

@fastify/middie is the Express-style middleware engine for Fastify. In versions from 9.0.0 through 9.3.5, the use() function joined a path-scoped middleware's mount path to the plugin scope prefix in a way that could diverge from the path Fastify's router serves, most often when the scope prefix ended in a slash, and also for array mount paths and for mount paths written without a leading slash. When the two paths disagreed, the middleware was silently skipped while the route handler still ran, so path-scoped middleware such as authentication, authorization, or rate limiting could be bypassed by a single unauthenticated request. The issue is fixed in @fastify/middie 9.3.6, and users should upgrade to 9.3.6 or later. As a workaround, enforce authorization in a Fastify hook such as preHandler that runs after the router has resolved the request.

• 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.