---
title: "CVE-2026-33807: @fastify/express's middleware path doubling causes authentication bypass in child plugin scopes"
url: https://erroneo.us/security/cve/CVE-2026-33807/
description: "CVE-2026-33807 (@fastify/express (npm)): @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-15
date_modified: 2026-04-16
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-33807"
kev: false
cvss: 9.1
epss: 0.00534
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-33807: @fastify/express's middleware path doubling causes authentication bypass in child plugin scopes

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.5% , percentile 44: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 44% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes @fastify/express (npm) <= 4.0.4, fixed in 4.0.5 . Check your lockfile, including transitive dependencies.

## What it is

@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.

## Affected packages

- npm `@fastify/express` <= 4.0.4 → fixed: 4.0.5

## Affected products (CVE record)

- fastify @fastify/express: < 4.0.5

## Severity

- openjs CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`
- GitHub CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.53% probability of exploitation activity in the next 30 days; percentile 44.

## Sources

- [GitHub Advisory GHSA-hrwm-hgmj-7p9c](https://github.com/advisories/GHSA-hrwm-hgmj-7p9c)
- [CVE.org record CVE-2026-33807](https://www.cve.org/CVERecord?id=CVE-2026-33807)
- [NVD entry CVE-2026-33807](https://nvd.nist.gov/vuln/detail/CVE-2026-33807)
- [github.com](https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c)
- [cna.openjsf.org](https://cna.openjsf.org/security-advisories.html)

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Source page: https://erroneo.us/security/cve/CVE-2026-33807/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt