CVE-2026-33804
HighSummary
Versions @fastify/middie 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization, allowing requests with duplicate slashes to bypass authentication and authorization checks.
Risk Assessment
Organizations may be exposed to unauthorized access if applications use the deprecated ignoreDuplicateSlashes option, potentially leading to serious security breaches.
Recommendation
It is recommended to upgrade to @fastify/middie version 9.3.2 to fix this issue. Alternatively, the ignoreDuplicateSlashes option can be disabled.
Other vulnerabilities in @fastify/middie
See all- CVE-2026-22031High
The @fastify/middie plugin prior to version 9.1.0 has a vulnerability that allows bypassing registered middleware using URL-encoded characters. Attackers can access protected endpoints without the constraints imposed by the middleware.
- CVE-2026-14198Critical
A vulnerability in @fastify/middie versions 9.1.0 through 9.3.2 causes a mismatch in handling encoded slashes (%2F) between the middleware and Fastify's router. An attacker can bypass middleware used for authentication, authorization, rate limiting, or auditing by sending a crafted request with an encoded slash in the parameter position.
- CVE-2026-6270Critical
Versions of @fastify/middie up to 9.3.1 do not register inherited middleware directly on child plugin engine instances. This allows unauthenticated requests to bypass authentication and authorization checks for routes defined in child plugins.
- CVE-2026-2880Critical
A vulnerability in @fastify/middie versions < 9.2.0 can result in authentication/authorization bypass when using path-scoped middleware. When Fastify router normalization options are enabled, crafted request paths may bypass middleware checks.
- CVE-2026-14181High
Vulnerability in @fastify/middie versions 9.1.0 through 9.3.2 causes Node.js process termination when processing malformed percent-encoded sequences in request paths. The issue only affects the standalone engine API (middie.run), not the Fastify plugin path.
Original NVD description (English source)
@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.