CVE-2026-14198CWE-436

@fastify/middie vulnerable to authorization bypass via encoded slash in path parameter values

Critical · published July 1, 2026

CVSS v3.1
9.1
EPSS
1%
Percentile
43.1
In the wild
Unconfirmed
What it is

@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 record
Technical detail
CVSS v3.1
9.1 · CRITICAL
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
CVSS v4.0
Not supplied
EPSS
0.00517 · 43.1th percentile
Weakness
CWE-436 · Interpretation Conflict
Published
2026-07-01T11:29Z
EPSS history
Timeline
  • 01 JUL 11:29Z
    @fastify/middie vulnerable to authorization bypass via encoded slash in path parameter values
    cvelistv5