@fastify/express Authentication Bypass via URL Normalization Gaps

A critical vulnerability exists in @fastify/express version 4.0.4 related to URL normalization. When Fastify router normalization options (ignoreDuplicateSlashes or useSemicolonDelimiter) are enabled, the @fastify/express module fails to properly normalize URLs before passing them to Express middleware. This discrepancy allows attackers to bypass authentication and authorization checks implemented in Express middleware. The vulnerability stems from the enhanceRequest function in index.js, which doesn't normalize duplicate slashes or strip semicolon-delimited parameters, leading to a mismatch between Fastify's route matching and Express's middleware handling. This issue is distinct from CVE-2026-22037, which addressed percent-encoding bypass. Successful exploitation grants unauthenticated access to protected resources, impacting applications that rely on Express middleware for security.

Attack Chain

1. An attacker identifies a protected route, such as /admin/dashboard, that is secured by Express middleware within a Fastify application using @fastify/express.
2. The attacker crafts a malicious URL containing either duplicate slashes (e.g., //admin/dashboard) or semicolon delimiters (e.g., /admin;bypass).
3. The attacker sends an HTTP request to the target server using the crafted malicious URL.
4. Fastify's router, configured with ignoreDuplicateSlashes: true or useSemicolonDelimiter: true, normalizes the URL for route matching.
5. @fastify/express's enhanceRequest function passes the original, un-normalized URL to the Express middleware.
6. The Express middleware, expecting the normalized URL (e.g., /admin/dashboard), fails to match the malicious URL (e.g., //admin/dashboard or /admin;bypass).
7. The authentication/authorization checks in the Express middleware are bypassed.
8. The Fastify route handler executes, granting the attacker unauthorized access to the protected resource, potentially leading to data breaches or other malicious activities.

Impact

This vulnerability allows for complete authentication bypass in applications that utilize Express middleware for path-based access control and are built on @fastify/express. Successful exploitation leads to unauthorized access to protected resources like admin panels, APIs, and sensitive user data. The duplicate slash vector impacts applications with ignoreDuplicateSlashes: true, while the semicolon vector affects those with useSemicolonDelimiter: true. This bypass affects all Express middleware using prefix path matching, including popular authentication, authorization, and rate-limiting packages. The root cause lies in the unexpected behavior of @fastify/express and its interaction with Fastify's router options, potentially affecting a large number of applications.

Recommendation

• Deploy the Sigma rule Detect Fastify Express Double Slash Authentication Bypass to detect attempts to exploit the duplicate slash vulnerability via web server logs.
• Deploy the Sigma rule Detect Fastify Express Semicolon Authentication Bypass to detect attempts to exploit the semicolon delimiter vulnerability via web server logs.
• Upgrade to a patched version of @fastify/express when available. This should include URL normalization before passing to Express middleware as suggested in the advisory.
• Review Fastify configurations to understand if ignoreDuplicateSlashes: true or useSemicolonDelimiter: true are actively used and assess the impact of disabling them until a patch is applied.