CVE-2026-22037: fastify fastify-express

Vulnerability summary

CVE-2026-22037 is a high (CVSS 8.4) vulnerability in fastify fastify-express. EPSS estimates a 0.35% chance of exploitation in the next 30 days, higher than 27.1% of scored vulnerabilities.

Timeline

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

Description

The @fastify/express plugin adds full Express compatibility to Fastify. A security vulnerability exists in @fastify/express prior to version 4.0.3 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. The vulnerability is caused by how @fastify/express matches requests against registered middleware paths. This vulnerability is similar to, but differs from, CVE-2026-22031 because this is a different npm module with its own code. Version 4.0.3 of @fastify/express contains a patch fort 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.
• CWE-288Authentication Bypass Using an Alternate Path or ChannelThe product requires authentication, but the product has an alternate path or channel that does not require authentication.