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CVE-2026-6270

Fastify Fastify/middie vulnerability

9.1 CRITICALPublic exploitEPSS 0.61%
  1. Published16 Apr 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

Fastify Fastify/middie< 9.3.2

Remediation

Fixed in
Fastify Fastify/middie≥ 9.3.2

References

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Frequently asked

Is CVE-2026-6270 being actively exploited?

CVE-2026-6270 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.61%.

How severe is CVE-2026-6270?

CVE-2026-6270 has a CVSS 3.1 base score of 9.1, rated critical. EPSS estimates a 0.61% probability of exploitation.

Which products are affected by CVE-2026-6270?

Affected products include Fastify Fastify/middie.

How do I fix CVE-2026-6270?

Upgrade affected software to a fixed version, for example Fastify Fastify/middie 9.3.2 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.