GHSA-v9ww-2j6r-98q6
HIGHGHSA-v9ww-2j6r-98q6 is a high-severity (CVSS 7.4) CWE-436 vulnerability in @fastify/middie. O3 Security confirms whether GHSA-v9ww-2j6r-98q6 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
@fastify/middienpmDescription
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 }.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @fastify/middie | all versions | 9.3.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @fastify/middie. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update @fastify/middie to 9.3.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-v9ww-2j6r-98q6 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-v9ww-2j6r-98q6 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-v9ww-2j6r-98q6. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-v9ww-2j6r-98q6 in your dependencies?
O3 detects GHSA-v9ww-2j6r-98q6 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.