Fastify Middie Middleware Path Bypass

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Summary

CVECVE-2026-22031
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-01-19 16:15:54 UTC
Updated2026-05-14 15:41:44 UTC
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.

Risk And Classification

Primary CVSS: v3.1 8.8 HIGH from [email protected]

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

EPSS: 0.001440000 probability, percentile 0.342130000 (date 2026-05-25)

Problem Types: CWE-177 | CWE-177 CWE-177: Improper Handling of URL Encoding (Hex Encoding)

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary8.8HIGHCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
3.1[email protected]Secondary8.4HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L
3.1CNADECLARED8.4HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Fastify Fastify/middie All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Fastify Middie affected < 9.1.0 Not specified

References

ReferenceSourceLinkTags
github.com/fastify/middie/pull/245 [email protected] github.com Issue Tracking, Patch
github.com/fastify/middie/commit/d44cd56eb724490babf7b452fdbbdd37ea2effba [email protected] github.com Patch
github.com/fastify/middie/releases/tag/v9.1.0 [email protected] github.com Product, Release Notes
github.com/fastify/middie/security/advisories/GHSA-cxrg-g7r8-w69p [email protected] github.com Exploit, Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis
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