---
title: "CVE-2026-6270: @fastify/middie vulnerable to middleware authentication bypass in child plugin scopes"
url: https://erroneo.us/security/cve/CVE-2026-6270/
description: "CVE-2026-6270 (@fastify/middie (npm)): @fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-16
date_modified: 2026-04-16
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-6270"
kev: false
cvss: 9.1
epss: 0.00608
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-6270: @fastify/middie vulnerable to middleware 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 0.6% , percentile 48: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 48% 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/middie (npm) <= 9.3.1, fixed in 9.3.2 . Check your lockfile, including transitive dependencies.

## What it is

@fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. When a Fastify application registers authentication middleware in a parent scope and then registers child plugins with @fastify/middie, the child scope does not inherit the parent middleware. This allows unauthenticated requests to reach routes defined in child plugin scopes, bypassing authentication and authorization checks. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds.

## Affected packages

- npm `@fastify/middie` <= 9.3.1 → fixed: 9.3.2

## Affected products (CVE record)

- @fastify/middie @fastify/middie: < 9.3.2

## 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 no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.61% probability of exploitation activity in the next 30 days; percentile 48.

## Sources

- [GitHub Advisory GHSA-72c6-fx6q-fr5w](https://github.com/advisories/GHSA-72c6-fx6q-fr5w)
- [CVE.org record CVE-2026-6270](https://www.cve.org/CVERecord?id=CVE-2026-6270)
- [NVD entry CVE-2026-6270](https://nvd.nist.gov/vuln/detail/CVE-2026-6270)
- [github.com](https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c)
- [github.com](https://github.com/fastify/middie/security/advisories/GHSA-72c6-fx6q-fr5w)
- [cna.openjsf.org](https://cna.openjsf.org/security-advisories.html)

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Source page: https://erroneo.us/security/cve/CVE-2026-6270/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt