CVE-2026-22031

Fastify Middie Middleware Path Bypass

@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.

Affected

fastify · middie

CVSS v3.1 NVD

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

CVSS v3.1 GITHUB

8.4 HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L

EPSS

0.53% chance of exploitation in the next 30 days, 44th percentile

Published

2026-01-19

Updated

2026-01-20

Proof-of-concept exploits (1)

• advisories/GHSA-cxrg-g7r8-w69p

Security advisories

• fastify/middie/security/advisories/GHSA-cxrg-g7r8-w69p
• fastify/middie/pull/245

References

• CVE Record
• NVD entry