18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Versions prior to 3.9.3 allow an unauthenticated HTTP client to pollute Object.prototype in the Node.js process hosting the middleware, via two unvalidated entry points that reach internal object-key writes: getResourcesHandler and missingKeyHandler. This can break authorisation checks (if (user.isAdmin) returning true for any user), cause type-confusion DoS, and depending on downstream code it can be chained into RCE.

Affected: npm: i18next-http-middleware

### 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 pla…