CVE-2026-6270: CVE-2026-6270: Authentication Bypass via Middleware Interpretation Conflict in Fastify Middie

CVE-2026-6270: Authentication Bypass via Middleware Interpretation Conflict in Fastify Middie

Vulnerability ID: CVE-2026-6270
CVSS Score: 9.1
Published: 2026-04-16

CVE-2026-6270 is a critical authentication and authorization bypass vulnerability affecting the Fastify ecosystem, specifically @fastify/middie and @fastify/fastify-express. The flaw arises from an interpretation conflict in middleware path propagation, allowing unauthenticated requests to bypass security guards and access protected child plugin routes.

TL;DR

Authentication bypass in Fastify middie <= 9.3.1 due to middleware path propagation errors.

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⚠️ Exploit Status: POC

Technical Details

• CWE ID: CWE-436
• Attack Vector: Network
• CVSS v3.1 Score: 9.1 (Critical)
• Impact: Authentication and Authorization Bypass
• Exploit Status: Proof of Concept (PoC) Available
• KEV Status: Not Listed

Affected Systems

• @fastify/middie <= 9.3.1
• @fastify/fastify-express <= 4.0.4
• Node.js server applications utilizing Fastify with nested plugin architectures
• @fastify/middie: <= 9.3.1 (Fixed in: 9.3.2)
• @fastify/fastify-express: <= 4.0.4 (Fixed in: Dependent on middie update)

Code Analysis

Commit: ca42dd7

Fix direct engine middleware registration bypassing scope propagation prefixes.

Commit: 29162c8

Implement resolveNormalizationOptions to synchronize path matching configurations.

Exploit Details

• GitHub Security Advisory: Proof of concept code documenting the duplicate slash bypass via path normalization configurations.

Mitigation Strategies

• Upgrade @fastify/middie to version 9.3.2.
• Enforce dependency resolutions for @fastify/fastify-express to ensure patched middie versions.
• Migrate security controls from Express middleware to native Fastify lifecycle hooks (e.g., preHandler).

Remediation Steps:

1. Run npm audit to identify vulnerable versions of @fastify/middie in the dependency tree.
2. Update package.json to require @fastify/middie@^9.3.2.
3. If using fastify-express, use npm overrides or yarn resolutions to force the sub-dependency update.
4. Rebuild the application and run unit tests ensuring route accessibility remains properly restricted.
5. Deploy the patched application to production environments.

References

• Official CVE Record
• NVD Entry
• OpenJS Foundation Advisories

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Read the full report for CVE-2026-6270 on our website for more details including interactive diagrams and full exploit analysis.