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

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.6%, percentile 48: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 48% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes @fastify/middie (npm) <= 9.3.1, fixed in 9.3.2. Check your lockfile, including transitive dependencies.

What it is

@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.

From the GitHub advisory

Impact

@fastify/middie v9.3.1 and earlier incorrectly re-prefixes middleware paths when propagating them to child plugin scopes. When a child plugin is registered with a prefix that overlaps with a parent-scoped middleware path, the middleware path is modified during inheritance and silently fails to match incoming requests.

This results in complete bypass of middleware security controls for all routes defined within affected child plugin scopes, including nested (grandchild) scopes. Authentication, authorization, rate limiting, and any other middleware-based security mechanisms are skipped. No special request crafting or configuration is required.

This is the same vulnerability class as GHSA-hrwm-hgmj-7p9c (CVE-2026-33807) in @fastify/express.

Patches

Weakness type: CWE-436.

Product: @fastify/middie (npm) · Assigned by: openjs · Published: 2026-04-16

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.61% probability of exploitation activity in the next 30 days; percentile 48.

Timeline

• 2026-04-16 — CVE record last updated
• 2026-04-16 — CVE record published by openjs
• 2026-04-16 — GitHub-reviewed advisory GHSA-72c6-fx6q-fr5w published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-72c6-fx6q-fr5w github.com
2. CVE.org record CVE-2026-6270 cve.org
3. NVD entry CVE-2026-6270 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. cna.openjsf.org cna.openjsf.org

Critical npm advisories · Today’s Security Digest