GitHub Advisoryadvisory · npm

GHSA-v9ww-2j6r-98q6 @fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option

HIGH Published 2026-04-16 Modified 2026-04-16 Official page ↗
Aliases
CVE-2026-33804
Ecosystem
npm
Packages
@fastify/middie
Severity
High
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Weaknesses
CWE-436
Reviewed
2026-04-16

Affected packages

• npm @fastify/middie: < 9.3.2 → fixed in 9.3.2

Description

Impact `@fastify/middie` v9.3.1 and earlier does not read the deprecated (but still functional) top-level `ignoreDuplicateSlashes` option, only reading from `routerOptions`. This creates a normalization gap: Fastify's router normalizes duplicate slashes but middie does not, allowing middleware bypass via URLs with duplicate leading slashes (e.g., `//admin/secret`). This only affects applications using the deprecated top-level configuration style (`fastify({ ignoreDuplicateSlashes: true })`). Applications using `routerOptions: { ignoreDuplicateSlashes: true }` are not affected. This is distinct from GHSA-8p85-9qpw-fwgw (CVE-2026-2880), which was patched in v9.2.0. Patches Upgrade to `@fastify/middie` >= 9.3.2. Workarounds Migrate from deprecated top-level `ignoreDuplicateSlashes: true` to `routerOptions: { ignoreDuplicateSlashes: true }`.

Related

Alias · CVE (1)

  • CVE-2026-33804 @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is… vulnerability

Weakness · CWE (1)

  • CWE-436 Interpretation Conflict weakness

References