fastify/middie · Security
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
- GHSA ID
- GHSA-v9ww-2j6r-98q6
- CVE ID
- CVE-2026-33804
- Severity
- high
- Published
- Updated
- CVSS
- 7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
- CVSS v3
- 7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
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 }.
CWEs
- CWE-436 · Interpretation Conflict