CVE-2026-14198
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk23th percentile - higher than 23% of all known CVEs
Summary
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.
Risk Assessment
The organization is at risk of unauthorized access to protected resources, potentially leading to data breaches, privilege escalation, or DoS attacks on sensitive endpoints.
Recommendation
Upgrade @fastify/middie to version 9.3.3 immediately. If upgrading is not possible, avoid using parameterized middleware paths for security decisions and enforce authentication at the route handler or via a Fastify hook that runs after the router has resolved the request.
Other vulnerabilities in @fastify/middie
See all- CVE-2026-33804High
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.
- 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-2026-85184Critical
In @fastify/middie versions >= 9.1.0 and before 9.3.4, the decision to run path-scoped middleware is made by matching against the raw request target, while the Fastify router resolves an absolute-form request target to its path before dispatching. Because the two layers evaluate different strings, a request using an absolute-form target reaches the route handler while the path-scoped middleware, such as authentication or authorization, is skipped. An unauthenticated network attacker can use this to bypass path-based access controls in a Fastify application that relies on middie for those controls.
- 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 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching middleware paths, while Fastify's underlying router preserves the encoding during route lookup. The two layers disagree on the canonical request path, so the middleware fails to match a URL that the route handler does match. When middleware is used for authentication, authorization, rate limiting, or auditing on parameterized paths, an attacker can reach the protected handler by sending a single crafted URL with an encoded slash in the parameter position. The bypass is HTTP method agnostic and requires no authentication or special preconditions. Patches: upgrade to @fastify/middie 9.3.3. Workarounds: avoid parameterized middleware paths for security decisions, or enforce authentication at the route handler or via a Fastify hook that runs after the router has resolved the request.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS