fastify middie up to 9.3.2 URL position. interpretation conflict

Summaryinfo

A vulnerability categorized as critical has been discovered in fastify middie up to 9.3.2. The affected element is an unknown function of the component URL Handler. Executing a manipulation of the argument position. can lead to interpretation conflict. This vulnerability is tracked as CVE-2026-14198. The attack can be launched remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability has been found in fastify middie up to 9.3.2 and classified as critical. This vulnerability affects an unknown functionality of the component URL Handler. The manipulation of the argument position. with an unknown input leads to a interpretation conflict vulnerability. The CWE definition for the vulnerability is CWE-436. Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

@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.

The advisory is available at github.com. This vulnerability was named CVE-2026-14198 since 06/30/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Technical details are known, but there is no available exploit.

Upgrading to version 9.3.3 eliminates this vulnerability.

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Productinfo

Vendor

• fastify

Name

• middie

Version

• 9.3.0
• 9.3.1
• 9.3.2

Website

• Product: https://github.com/fastify/middie/

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