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CVE-2026-6270 — Vulnerability in @fastify/middie — CVE Details

CRITICAL

@fastify/middie vulnerable to middleware authentication bypass in child plugin scopes

Published April 16, 2026Updated July 8, 2026Source: osv

Summary

### 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](https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c) (CVE-2026-33807) in `@fastify/express`. ### Patches Upgrade to `@fastify/middie` v9.3.2 or later. ### Workarounds None. Upgrade to the patched version.

Advisory Timeline

  • First published April 16, 2026 (179 days ago).
  • Last revised July 8, 2026 — 82 days after initial publication.

Remediation

Upgrade to the fixed version using your package manager.

npm
Update @fastify/middie to 9.3.2 or later
npm install @fastify/middie@9.3.2

After upgrading, run your dependency scanner again to confirm the vulnerability is resolved.

Affected Packages (1)

Affects 1 ecosystem: npm (1).

PackageEcosystemAffectedFixed In
@fastify/middie
npm
All versions9.3.2

Vulnerability Classification

Common Weakness Enumeration (CWE) identifiers for this vulnerability type.

CVSS Score Breakdown

What the CVSS (Common Vulnerability Scoring System) 9.1 score means for each attack dimension.

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

References

Frequently Asked Questions

What is CVE-2026-6270?
@fastify/middie vulnerable to middleware authentication bypass in child plugin scopes This vulnerability has been assigned a severity rating of CRITICAL (CVSS score: 9.1/10).
How do I check if my project is affected by CVE-2026-6270?
CVE-2026-6270 affects @fastify/middie. Use GeekWala's free vulnerability scanner to check your dependencies against CVE-2026-6270 and 200,000+ other known vulnerabilities.

Severity & Exploitability

CVSS Score
9.1

Exploitation is straightforward and causes maximum impact. Patch immediately.

Also Known As

GHSA-72c6-fx6q-fr5w

Related CVEs

  • CVE-2026-33808
    CRITICAL

    @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

  • CVE-2026-33805
    CRITICAL

    Fastify's connection header abuse enables stripping of proxy-added headers

  • CVE-2026-33807
    CRITICAL

    @fastify/express's middleware path doubling causes authentication bypass in child plugin scopes

  • CVE-2026-107722
    CRITICAL

    fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion

  • CVE-2026-108261
    CRITICAL

    TinaCMS admin preview iframe loads an attacker-controlled origin from the URL fragment

More npm advisories

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