CVE-2026-33804: Fastify Middie Auth Bypass Vulnerability

CVE-2026-33804 Overview

CVE-2026-33804 is an Authorization Bypass vulnerability affecting @fastify/middie versions 9.3.1 and earlier. The vulnerability allows attackers to bypass middleware authentication and authorization checks when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic fails to account for duplicate slash normalization performed by Fastify's router, creating a path-based security bypass condition.

Critical Impact
Attackers can bypass authentication and authorization middleware by crafting requests with duplicate slashes, potentially gaining unauthorized access to protected routes and sensitive resources.

Affected Products

@fastify/middie versions 9.3.1 and earlier
Fastify applications using the deprecated ignoreDuplicateSlashes option
Node.js web applications relying on @fastify/middie for middleware integration

Discovery Timeline

April 16, 2026 - CVE CVE-2026-33804 published to NVD
April 16, 2026 - Last updated in NVD database

Technical Details for CVE-2026-33804

Vulnerability Analysis

This vulnerability is classified under CWE-436 (Interpretation Conflict), where the middleware path matching logic interprets request paths differently than Fastify's core router. When the deprecated ignoreDuplicateSlashes option is enabled, Fastify's router normalizes duplicate slashes in the request path (e.g., //admin//dashboard becomes /admin/dashboard). However, the @fastify/middie middleware evaluates the original, non-normalized path before routing occurs.

This interpretation conflict creates a dangerous security gap where middleware-based authentication and authorization checks can be completely bypassed. An attacker can craft requests with strategically placed duplicate slashes that fail to match protected path patterns in the middleware, while still reaching the intended route handler after Fastify normalizes the path.

Root Cause

The root cause lies in the order of operations and the inconsistent path handling between @fastify/middie and Fastify's core router. When ignoreDuplicateSlashes is enabled, the middleware receives the raw request path with duplicate slashes intact, while the router processes a normalized version. This desynchronization allows paths like //api//protected//resource to slip past middleware rules targeting /api/protected/resource.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending HTTP requests with duplicate slashes in the URL path. For example, if middleware is configured to require authentication for requests matching /admin/*, a request to //admin/users or /admin//users may bypass the middleware check entirely while still being routed to the protected /admin/users endpoint by Fastify's router.

The vulnerability mechanism works as follows: middleware path matching checks the raw request path containing duplicate slashes, which does not match the expected pattern. The request proceeds without triggering authentication middleware. Fastify's router then normalizes the path by removing duplicate slashes and routes the request to the intended protected endpoint. See the GitHub Security Advisory GHSA-v9ww-2j6r-98q6 for additional technical details.

Detection Methods for CVE-2026-33804

Indicators of Compromise

HTTP request logs showing URLs with unexpected duplicate slashes (e.g., //api//, /admin//resource)
Access to protected endpoints from unauthenticated sessions or unauthorized users
Web server logs indicating path normalization occurring after middleware processing
Authentication middleware logs showing fewer requests than expected for protected routes

Detection Strategies

Review application configurations to identify usage of the deprecated ignoreDuplicateSlashes option in Fastify
Audit dependency versions to identify @fastify/middie installations at version 9.3.1 or earlier
Implement web application firewall (WAF) rules to detect and block requests containing consecutive slashes
Monitor authentication and authorization middleware logs for unexpected path patterns

Monitoring Recommendations

Enable detailed request logging that captures the original URL path before any normalization
Set up alerts for access to sensitive endpoints that bypass expected authentication flows
Implement anomaly detection for requests with unusual path patterns reaching protected resources
Regularly audit middleware execution logs against actual route handler invocations

How to Mitigate CVE-2026-33804

Immediate Actions Required

Upgrade @fastify/middie to version 9.3.2 or later immediately
Audit your Fastify configuration and disable the deprecated ignoreDuplicateSlashes option if currently enabled
Review access logs for evidence of exploitation attempts using duplicate slash patterns
Implement input validation to reject or normalize requests with duplicate slashes at the edge

Patch Information

The vulnerability is fixed in @fastify/middie version 9.3.2. This patch ensures the middleware path matching logic properly accounts for the duplicate slash normalization behavior when ignoreDuplicateSlashes is enabled. Organizations should update their package.json dependencies and run npm update @fastify/middie or the equivalent command for their package manager. For more information, refer to the GitHub Security Advisory GHSA-v9ww-2j6r-98q6 and the OpenJS Foundation Security Advisories.

Workarounds

Disable the deprecated ignoreDuplicateSlashes option in Fastify configuration (this is the only vendor-recommended workaround)
Implement a pre-handler hook that normalizes paths before middleware evaluation as a temporary measure
Deploy a reverse proxy or WAF rule to normalize or reject requests with duplicate slashes before they reach the application
Consider implementing route-level authentication checks as an additional layer of defense

bash

# Update @fastify/middie to patched version
npm update @fastify/middie@9.3.2

# Or explicitly install the patched version
npm install @fastify/middie@9.3.2

# Verify installed version
npm list @fastify/middie