CVE-2026-44373 is a medium-severity (CVSS 5.3) Path Traversal vulnerability in nitro. A fix is available for nitro — see the affected versions and patch details below.
Nitro: Proxy scope bypass via percent-encoded path traversal in `routeRules`
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-44373.
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-2026-44373 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.
nitronpmnitropacknpmDescription
A proxy route rule like:
routeRules: {
"/api/orders/**": { proxy: { to: "http://upstream/orders/**" } }
}
is intended to limit the proxy to URLs under /api/orders/. Before the patch, an attacker could bypass that scope by sending percent-encoded path traversal (..%2f) in the URL, causing Nitro to forward a request that the upstream resolved outside the configured scope. Example exploit:
GET /api/orders/..%2fadmin%2fconfig.json
Nitro sees ..%2f as opaque characters at match time, the /api/orders/** rule matched, and the raw path was forwarded to the upstream as /orders/..%2fadmin/config.json. An upstream that decodes %2F to / then resolved .. and can serve /admin/config.json outside the intended scope.
Are you affected?
Users may be affected if ALL of the following are true:
- Their project uses Nitro's
routeRuleswith aproxyentry ({ proxy: { to: "..." } }). - The proxy
tovalue uses a/**wildcard suffix to forward sub-paths. - The upstream behind the proxy decodes
%2Fas/before routing or filesystem lookup. - Proxy route rules are not handled natively at CDN (nitro v3 and vercel)
Whether the bypass actually leaks data depends on the upstream. Modern JS frameworks keep %2F opaque per RFC 3986 and are safe by construction.
- Safe examples: H3 v2, Express v5, Hono v4 — modern JS frameworks keep
%2Fopaque per RFC 3986. - Vulnerable examples: naive imlementations that decodes the URL, static file servers, CGI dispatchers, Python
os.path-based routing, anything sitting behind another layer that decodes%2F(common in microservice meshes).
Impact
Any HTTP path reachable from the Nitro server to the upstream could be requested, regardless of the configured /** scope. In typical deployments (API gateway, BFF, microservice proxy) this could expose internal admin endpoints, secrets endpoints, or other services the developer believed the scope rule fenced off.
Patched versions
Upgrade to one of:
- 2.13.4 or later (https://github.com/nitrojs/nitro/pull/4223)
- 3.0.260429-beta or later (https://github.com/nitrojs/nitro/pull/4222)
The fix canonicalizes the incoming pathname before building the upstream URL and rejects requests with 400 Bad Request if the resolved path would escape the rule's base. The bytes forwarded upstream are unchanged when the request is allowed.
Note: the fix assumes the upstream does not double-decode percent-encoding. If your upstream decodes twice (
%252F → %2F → /), it remains your responsibility to harden it. Single-decode is standard.
Credits
Reported by @mHe4am (@he4am on HackerOne) via the Vercel Open Source program.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | nitro | all versions | 3.0.260429-betanpm install nitro@3.0.260429-beta |
| 📦npm | nitropack | all versions | 2.13.4npm install nitropack@2.13.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nitro, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nitro to 3.0.260429-beta or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-44373 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-2026-44373 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-44373. 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-2026-44373 in your dependencies?
O3 Security finds CVE-2026-44373 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.