---
title: "CVE-2026-33808: @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)"
url: https://erroneo.us/security/cve/CVE-2026-33808/
description: "CVE-2026-33808 (@fastify/express (npm)): Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-15
date_modified: 2026-06-09
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-33808"
kev: false
cvss: 9.1
epss: 0.00551
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-33808: @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

**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 45: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 45% 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

Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled. In both cases, Fastify router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. An unauthenticated attacker can access protected routes by manipulating the URL path. PatchesUpgrade 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 4.0: 9.1 `CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`
- GitHub CVSS 4.0: 9.1 `CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA: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): 0.55% probability of exploitation activity in the next 30 days; percentile 45.

## Sources

- [GitHub Advisory GHSA-6hw5-45gm-fj88](https://github.com/advisories/GHSA-6hw5-45gm-fj88)
- [CVE.org record CVE-2026-33808](https://www.cve.org/CVERecord?id=CVE-2026-33808)
- [NVD entry CVE-2026-33808](https://nvd.nist.gov/vuln/detail/CVE-2026-33808)
- [github.com](https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fj88)
- [cna.openjsf.org](https://cna.openjsf.org/security-advisories.html)

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Source page: https://erroneo.us/security/cve/CVE-2026-33808/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt