Vulnerability Report

CVE-2026-33807

Title: Path Handling Bug Leads to Middleware Bypass in @fastify/express

Other

Proof Of Concept

No public PoC currently indexed for CVE-2026-33807.

CWE Category CWE-436

Published Date Apr 15, 2026

Modified Date Jun 01, 2026

Exploit Status Not Found

Score 9.1 CVSS v3.1

Exploit Probability (EPSS)

0.04%

Vulnerability Summary

CVE-2026-33807: @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required. Upgrade to @fastify/express v4.0.5 or later.

Impacted Vendors

fastify 1

Reference Links

https://cna.openjsf.org/security-advisories.html https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c

CVSS v3.1

Source Entity ce714d77-add3-4f53-aff5-83d477b104bb

Severity CRITICAL

9.1

Attack Vector

NETWORK

Complexity

LOW

Privileges

N/A

Interaction

NONE

Confidentiality

N/A

Integrity

N/A

Availability

N/A

Scope

UNCHANGED

RAW VECTOR CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Associated Attack Patterns (CAPEC)

Total: Patterns

CVE-2026-33807 Exploits & PoCs (Proof Of Concept)

No public PoCs found in our database for this CVE.

June 01, 2026 MODIFIED

Vulnerability data updated via NVD.

June 01, 2026 MODIFIED

Vulnerability data updated via NVD.

April 17, 2026 MODIFIED

Vulnerability data updated via NVD.

April 17, 2026 MODIFIED

Vulnerability data updated via NVD.

Attack Vector Matrix

Access Vector NETWORK

Complexity LOW

Privileges N/A

Interaction NONE

CVSS Vector String


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

Affected Stack

No specific products linked.