CVE-2026-22031: fastify fastify\/middie

Vulnerability summary

CVE-2026-22031 is a high (CVSS 8.8) vulnerability in fastify fastify\/middie. EPSS estimates a 0.53% chance of exploitation in the next 30 days, higher than 43.8% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 19-01-2026
   PublishedVulnerability first disclosed.
2. 20-01-2026
   Last modifiedVulnerability information updated.

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.

• CWE-177Improper Handling of URL Encoding (Hex Encoding)The product does not properly handle when all or part of an input has been URL encoded.