CVE-2026-112712 in fastify-middie
Summary
by MITRE • 10/21/2026
@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.
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Analysis
by VulDB Data Team • 10/21/2026
The vulnerability identified in fastify/middie versions ranging from 9.0.0 through 9.3.5 represents a critical logic error within the middleware mounting mechanism of this Express-style engine for Fastify applications. The core technical flaw stems from an incorrect path resolution strategy when combining plugin scope prefixes with specific mount paths defined by developers. Specifically, the use function improperly joins these two components, leading to a divergence between the actual URL pattern registered in Fastify's internal router and the effective path where fastify/middie applies its middleware logic. This discrepancy is most frequently triggered when the plugin scope prefix terminates with a trailing slash or when array-based mount paths are utilized without standard normalization practices such as ensuring a leading slash. Consequently, the middleware engine calculates an incorrect target route for execution, causing it to silently skip processing requests that match the intended logical path but not the miscalculated technical path.
The operational impact of this misalignment is severe because it creates a silent failure mode where security-critical middleware functions are bypassed without raising any errors or warnings. Path-scoped middleware typically handles essential security controls such as authentication, authorization checks, and rate limiting mechanisms designed to protect specific routes from unauthorized access or abuse. When the path mismatch occurs, these protective layers are effectively disabled for the affected endpoints because the router successfully matches the request against its own internal rules while the middleware engine fails to intercept it due to the divergent path calculation. This allows attackers to execute unauthenticated requests against protected resources by exploiting the gap between where Fastify expects the handler to run and where @fastify/middie actually applies its logic, thereby undermining the security posture of the application entirely.
This vulnerability aligns with CWE-284 Improper Access Control as it results in a failure to enforce required access restrictions due to flawed implementation logic rather than missing controls. Furthermore, from an adversary perspective, this flaw facilitates unauthorized access and potential privilege escalation by allowing attackers to bypass authentication mechanisms that are supposed to gatekeep sensitive endpoints. In the context of MITRE ATT&CK techniques, this scenario relates to T1078 Valid Accounts if it allows impersonation or T1546 Event Triggered Execution if used in conjunction with other exploits, though primarily it represents a fundamental breakdown in access control enforcement known as CWE-284. The silent nature of the failure makes detection difficult during standard testing unless specific path-based security audits are conducted to verify middleware execution flow against actual request paths.
To remediate this issue, organizations must upgrade @fastify/middie to version 9.3.6 or any later release where the path joining logic has been corrected to ensure consistency between plugin scope prefixes and mount paths regardless of trailing slashes or array formats. For environments that cannot immediately patch due to dependency constraints or testing requirements, a robust workaround involves shifting critical security checks out of the middleware layer entirely. Instead, developers should implement authorization and authentication validation within Fastify hooks such as preHandler which execute after the router has resolved the request path but before the route handler processes it. This ensures that access control decisions are based on the definitive routing context provided by Fastify itself, eliminating the ambiguity introduced by external middleware path resolution errors and maintaining a consistent security boundary regardless of how routes are defined or mounted.