CVE-2026-33804: @fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.
Other sources
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 }.
— GitHub
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-33804?
CVE-2026-33804 has a high severity rating due to the potential for middleware bypass in affected versions.
How do I fix CVE-2026-33804?
To fix CVE-2026-33804, upgrade @fastify/middie to version 9.3.2 or later, which addresses the vulnerability.
Which versions are affected by CVE-2026-33804?
CVE-2026-33804 affects @fastify/middie versions 9.3.1 and earlier.
What impact does CVE-2026-33804 have on applications?
CVE-2026-33804 can result in unauthorized access to middleware functions, potentially leading to security breaches.
Is the ignoreDuplicateSlashes option related to CVE-2026-33804?
Yes, the deprecation of the ignoreDuplicateSlashes option is directly linked to the vulnerability in CVE-2026-33804.