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Vulnerability record · CVE-2026-22031 · published 19 January 2026

CVE-2026-22031: Fastify\/middie vulnerability

Fastify · Fastify\/Middie

@fastify/middie is the plugin that adds middleware support on steroids to Fastify. A security vulnerability exists in @fastify/middie prior to version 9.1.0 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. Version 9.1.0 fixes the issue.

8.8 CVSS 3.1 High EPSS 0.53% · top 57.8% CWE-177 · CWE-177
8.8CVSS 3.1 base score
0.53%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References, 1 tagged exploit
17 Jun 2026Last modified by NVD

Description

@fastify/middie is the plugin that adds middleware support on steroids to Fastify. A security vulnerability exists in @fastify/middie prior to version 9.1.0 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. Version 9.1.0 fixes the issue.

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/fastify/middie/commit/d44cd56eb724490babf7b452fdbbdd37ea2effba Patch
https://github.com/fastify/middie/pull/245 Issue TrackingPatch
https://github.com/fastify/middie/releases/tag/v9.1.0 ProductRelease Notes
https://github.com/fastify/middie/security/advisories/GHSA-cxrg-g7r8-w69p ExploitVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.1CVE-2026-85184Fastify\/middie interpretation conflict vulnerability@fastify/middie versions >= 9.1.0 and before 9.3.4 decide whether to run path-scoped middleware by matching against the raw request target, while the…EPSS 0.52%9.1CVE-2026-14198Fastify\/middie interpretation conflict vulnerability@fastify/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching middleware paths, while Fastif…EPSS 0.52%9.1CVE-2026-33804Fastify\/middie interpretation conflict vulnerability@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. …EPSS 0.49%9.1CVE-2026-6270Fastify\/middie interpretation conflict vulnerability@fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. When a Fastify application…EPSS 0.61%8.2CVE-2026-2880Fastify\/middie improper input validation vulnerabilityA vulnerability in @fastify/middie versions < 9.2.0 can result in authentication/authorization bypass when using path-scoped middleware (for example,…EPSS 0.52%7.5CVE-2026-14181Fastify\/middie vulnerability@fastify/middie versions 9.1.0 through 9.3.2 fail to guard the URL normalization step used by the standalone engine when incoming request paths conta…EPSS 0.50%

Source: NIST National Vulnerability Database (record CVE-2026-22031), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.