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MEDIUM severity

CVE-2026-44373 — nitro

MEDIUMFix: nitrojs/nitro#4222

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`

Also known asGHSA-5w89-w975-hf9q
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

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

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs39th percentile — riskier than 39% of all scored CVEsHighest risk
0.00%0.36%0.72%1.07%0.0%0.5%Jun 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

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.

70other npm packages depend on this — each one inherits the vulnerability until it's patched upstream
nitronpm
8.4Mdownloads / week
nitropacknpm
3.5Mdownloads / week

Description

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:

  1. Their project uses Nitro's routeRules with a proxy entry ({ proxy: { to: "..." } }).
  2. The proxy to value uses a /** wildcard suffix to forward sub-paths.
  3. The upstream behind the proxy decodes %2F as / before routing or filesystem lookup.
  4. 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 %2F opaque 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:

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

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
📦npmnitroall versions3.0.260429-betanpm install nitro@3.0.260429-beta
📦npmnitropackall versions2.13.4npm install nitropack@2.13.4

Affected Products

1 product · 2 configurations
Application
nitronitro
> 2.13.4 && < 3.0.260429
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    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.

  2. 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.

  3. Workarounds

    Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Frequently Asked Questions

A proxy route rule like: ```ts 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 `
O3 Security · Impact-Aware SCA

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CVE-2026-44373: nitro Path Traversal — Fix Available