CVE-2026-6270: @fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engin

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CRITICAL 9.1REMOTEUPGRADE

CVE-2026-6270 is a critical-severity flaw (CVSS 9.1) affecting fastify\/middie. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

@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.

Why this priority

Critical severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 58%)
CISA exploited list	No
Weakness	CWE-436
Published	2026-04-16
Last modified	2026-06-17

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-16 14:16 UTCPublished CVE record published
2026-06-17 11:00 UTCUpdated Record last modified

fastify fastify\/middie UPGRADE

Affected: < 9.3.2

Fixed in: 9.3.2

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls fastifymiddie

2. UPGRADE to 9.3.2

npm install fastifymiddie@">=9.3.2"

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-6270?

fastify\/middie: upgrade to 9.3.2. Exact commands for each OS are on this page.

Is CVE-2026-6270 being exploited?

CVE-2026-6270 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-6270 affect?

CVE-2026-6270 affects fastify\/middie. Severity is critical (CVSS 9.1). @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 wit

How urgent is CVE-2026-6270?

Priority 52/100. Critical severity; Public exploit reference; Remotely exploitable, no login, no user action

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References

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