@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

• CVE-2026-33804: 0.28% estimated exploitation activity in the next 30 days; percentile 20.30 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 }.

Published by the source

16 Apr 2026, 22:28 UTC

Last modified by the source

16 Apr 2026, 22:56 UTC

Source last checked

26 Oct 2026, 12:48 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-v9ww-2j6r-98q6

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-v9ww-2j6r-98q6

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# Patch Next advisory brief: CVE-2026-33804

Advisory: @fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
IDs: CVE-2026-33804, GHSA-v9ww-2j6r-98q6
Record: https://patchnext.com/advisories/cve-2026-33804 (revision 2, source checked 26 Oct 2026, 12:48 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @fastify/middie 9.3.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @fastify/middie: affected <= 9.3.1; fixed in 9.3.2

## Suggested inspection (read-only)
- `npm ls @fastify/middie --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @fastify/middie.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-v9ww-2j6r-98q6 (OSV record GHSA-v9ww-2j6r-98q6 lists affected ranges: npm @fastify/middie >=0 <9.3.2 (fixed))
- Verified: https://github.com/advisories/GHSA-v9ww-2j6r-98q6 (GitHub-reviewed advisory GHSA-v9ww-2j6r-98q6 lists affected ranges: npm @fastify/middie >=0 <=9.3.1)

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