i18next-http-middleware: Prototype pollution and path traversal via user-controlled language and namespace parameters

Summary

Versions of i18next-http-middleware prior to 3.9.3 pass user-controlled lng and ns parameters to two internal paths that use them in ways that enable prototype pollution and, depending on the configured backend, path traversal or SSRF.

The vulnerable entry points are unauthenticated HTTP handlers that are part of the middleware's public API:

• getResourcesHandler — reads lng/ns from query parameters or route params and passes them unvalidated to:
  • utils.setPath(resources, [lng, ns], ...) — the setPath helper did not guard against __proto__, constructor, or prototype keys, writing into Object.prototype when those values were supplied.
  • i18next.services.backendConnector.load(languages, namespaces, ...) — depending on the configured backend, unvalidated path segments enabled filesystem path traversal (e.g. with i18next-fs-backend) or SSRF (e.g. with i18next-http-backend).
  • A namespaces.forEach(ns => i18next.options.ns.push(ns)) loop additionally performed permanent, unbounded growth of the shared singleton namespace list.
• missingKeyHandler — iterated the incoming request body with for...in, which traverses inherited prototype-chain properties. A POST body like {"__proto__": {"isAdmin": true}} was forwarded into saveMissing.

Impact

• Prototype pollution — a single unauthenticated request of the form GET /locales/resources.json?lng=__proto__&ns=isAdmin writes into Object.prototype, affecting every plain object created subsequently in the Node.js process. This can break authorization checks (if (user.isAdmin)), cause denial of service via type confusion, or be chained into RCE depending on what downstream code reads from polluted objects.
• Path traversal / SSRF — with filesystem or HTTP backends that interpolate lng/ns into paths or URLs, attacker-controlled values like ns=../../etc/passwd or lng=internal-service could reach resources outside the intended scope.
• Denial of service — the unbounded i18next.options.ns growth, plus repeated backend load calls, enabled memory and CPU exhaustion from unique namespace payloads.