CVE-2026-33804 — @fastify/middie
HIGHCVE-2026-33804 is a high-severity (CVSS 7.4) CWE-436 vulnerability in @fastify/middie. A fix is available for @fastify/middie — see the affected versions and patch details below.
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33804.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-33804 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@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 | 9.3.2npm install @fastify/middie@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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @fastify/middie to 9.3.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33804 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-33804 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-33804. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-33804 in your dependencies?
O3 Security finds CVE-2026-33804 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.