Middleware Bypass in Fastify Plugin by Fastify
CVE-2026-22037

8.4HIGH

Key Information:

Vendor

Fastify

Status
Fastify-express
Vendor
CVE Published:
19 January 2026

What is CVE-2026-22037?

A security issue exists in the @fastify/express plugin for Fastify, where middleware registered with specific path prefixes can be bypassed using URL-encoded characters. For example, the encoded URL /%61dmin can allow access to the /admin route, circumventing the middleware constraints due to a failure in matching the encoded path. This allows attackers to reach protected endpoints without the proper middleware checks. Version 4.0.3 of the @fastify/express plugin has released a patch to mitigate this vulnerability.

Affected Version(s)

fastify-express < 4.0.3

References

https://github.com/fastify/fastify-express/security/advis...
x_refsource_CONFIRM
https://github.com/fastify/fastify-express/commit/dc02a3f...
x_refsource_MISC

CVSS V3.1

Score:
8.4
Severity:
HIGH
Confidentiality:
High
Integrity:
High
Availability:
High
Attack Vector:
Network
Attack Complexity:
High
Privileges Required:
Low
User Interaction:
None
Scope:
Changed

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.