@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.
The @fastify/middie layer decodes the encoded slash %2F in path parameter values before matching against registered middleware paths. However, the Fastify router preserves the original encoding during route lookup. Both layers interpret the same path differently — the middleware does not recognize the request as matching its pattern and does not execute, while the route handler is invoked normally. An attacker only needs to send a crafted URL with %2F in place of a parameter — the HTTP method and additional preconditions are irrelevant.
An unauthenticated attacker can bypass middleware responsible for authentication, authorization, rate limiting, or auditing and gain direct access to a protected handler. This can result in violations of application data confidentiality and integrity.
Update @fastify/middie to version 9.3.3. As a temporary workaround: avoid using parameterized middleware paths for security decisions, and move authentication and authorization to the route handler or to a Fastify hook executed after route resolution by the router.
@fastify/middie in versions 9.1.0 to 9.3.2 (product: Fastify/Middie)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NFastify Fastify\/middie
APPFastify9.1.0 – 9.3.3 (excl.)
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