CVE-2026-33804
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
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](https://github.com/fastify/middie/security/advisories/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 }`.
How to fix CVE-2026-33804
To remediate CVE-2026-33804, upgrade the affected package to a fixed version below.
- —upgrade to 9.3.2 or later
Is CVE-2026-33804 being exploited?
Low — EPSS is 0.1%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 9.3.2
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.4 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N |