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. ### Affected versions `< 3.9.3`. ### Patch Fixed in **3.9.3**. The patch: 1. Blocks `__proto__`, `constructor`, and `prototype` keys in `utils.setPath`. 2. Replaces the `for...in` body iteration in `missingKeyHandler` with `Object.keys()` plus an explicit dangerous-keys guard. 3. Introduces a `utils.isSafeIdentifier` helper (denylist approach — still permits any legitimate i18next language code shape) that filters `lng`/`ns` values for path-traversal, path separators, control characters, prototype keys, and over-long inputs before they reach the backend connector and before they are pushed into `i18next.options.ns`. ### Workarounds No workaround short of upgrading. Front-proxying the middleware with a WAF rule that rejects requests containing `__proto__`, `constructor`, `prototype`, `..`, or control characters in `lng`/`ns` query parameters or body keys is a partial mitigation. ### Credits Discovered via an internal security audit of the i18next ecosystem.

Source published

22 Apr 2026 17:40:47 UTC

Source modified

13 May 2026 13:56:02 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Publisher rating · HIGH