CVE-2026-14198
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk23th percentile — higher than 23% of all known CVEs
Summary
A vulnerability in @fastify/middie versions 9.1.0 through 9.3.2 causes a mismatch in handling encoded slashes (%2F) between the middleware and Fastify's router. An attacker can bypass middleware used for authentication, authorization, rate limiting, or auditing by sending a crafted request with an encoded slash in the parameter position.
Risk Assessment
The organization is at risk of unauthorized access to protected resources, potentially leading to data breaches, privilege escalation, or DoS attacks on sensitive endpoints.
Recommendation
Upgrade @fastify/middie to version 9.3.3 immediately. If upgrading is not possible, avoid using parameterized middleware paths for security decisions and enforce authentication at the route handler or via a Fastify hook that runs after the router has resolved the request.
Original NVD description (English source)
@fastify/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching middleware paths, while Fastify's underlying router preserves the encoding during route lookup. The two layers disagree on the canonical request path, so the middleware fails to match a URL that the route handler does match. When middleware is used for authentication, authorization, rate limiting, or auditing on parameterized paths, an attacker can reach the protected handler by sending a single crafted URL with an encoded slash in the parameter position. The bypass is HTTP method agnostic and requires no authentication or special preconditions. Patches: upgrade to @fastify/middie 9.3.3. Workarounds: avoid parameterized middleware paths for security decisions, or enforce authentication at the route handler or via a Fastify hook that runs after the router has resolved the request.