What’s Vulnerable › Vulnerabilities

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

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-04-16 · updated 2026-06-17
• Weakness: CWE-436

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.

Affected products

• fastify middie: before 9.3.2 — fixed in 9.3.2

How to fix

Upgrade to a fixed version: fastify middie 9.3.2. Check the vendor advisory for mitigations.

References

• https://cna.openjsf.org/security-advisories.html
• https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c
• https://github.com/fastify/middie/security/advisories/GHSA-72c6-fx6q-fr5w

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