CVE-2026-85184: @fastify/middie path-scoped middleware bypass via absolute-form request

CVE-2026-85184 · Severity: critical · CVSS 9.1 · Published 2026-09-04

@fastify/middie is a Fastify plugin that manages Express-style middleware in web applications. Versions 9.1.0 through 9.3.3 have a flaw where path-scoped middleware (such as authentication or authorization) can be bypassed by attackers using absolute-form HTTP request targets. This allows unauthenticated attackers to access protected routes that should be guarded by the middleware, potentially exposing sensitive data or compromising application security.

Technologies: Fastify Middie. Vendors: Fastify.

Quick answers

Is CVE-2026-85184 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-85184 being exploited in the wild.
How severe is CVE-2026-85184?
Critical severity, with a CVSS score of 9.1, published 4 September 2026.
Which products and versions are affected?
OpenJS Foundation @fastify/middie >= 9.1.0, < 9.3.4.
Is there a fix for CVE-2026-85184?
A patch was published on 2026-09-04: Version 9.3.4 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-85184 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-85184?
Check whether you run Fastify Middie. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability is an interpretation conflict (CWE-436) where @fastify/middie evaluates path-scoped middleware matching against the raw request target, while the underlying Fastify router (find-my-way) resolves absolute-form targets to their path component before dispatching. An attacker sends a request with an absolute-form target like "GET http://anything/private/secrets" which bypasses middie's middleware check but matches the handler in the router, allowing access to protected routes. The attack requires only network access and no authentication. The vulnerability is patched in version 9.3.4 and later; workarounds involve enforcing authentication at the Fastify hook level (e.g., preHandler) which runs after route resolution.

Affected products

  • OpenJS Foundation @fastify/middie >= 9.1.0, < 9.3.4

Timeline

  • 2026-09-04: disclosed: CVE-2026-85184 published
  • 2026-09-04: patched: Version 9.3.4 released with fix

References

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