Vulnerability Details CVE-2026-22031
@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.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.001
EPSS Ranking 21.8%
CVSS Severity
CVSS v3 Score 8.4
References
Products affected by CVE-2026-22031
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cpe:2.3:a:openjsf:@fastify/middie:*