i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. In versions prior to 3.9.7, the missingKeyHandler blocked the literal request-body keys __proto__, constructor, and prototype (added in 3.9.3, see GHSA-5fgg-jcpf-8jjw), but did not reject dotted variants such as "__proto__.polluted". Downstream backends that split the missing-key string on a configured keySeparator (notably i18next-fs-backend ≤ 2.6.5) hand these keys to an unguarded setPath() walker that writes to Object.prototype. Applications that expose missingKeyHandler to untrusted input AND use i18next-fs-backend ≤ 2.6.5 are directly exploitable for remote prototype pollution. Other downstream backends that split the missing-key string the same way may be similarly affected. Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks. This issue has been fixed in version 3.9.7. If developers cannot upgrade immediately, they should do the following: do not expose missingKeyHandler to untrusted users (mount it behind authentication, or remove the route), add a request-body filter ahead of the handler that rejects any top-level key containing __proto__, constructor, or prototype after splitting on their configured keySeparator, and disable missing-key persistence (saveMissing: false) when accepting writes from untrusted input.

Affected: npm: i18next-http-middleware

### Impact

`i18next-http-middleware` ≤ 3.9.6's `missingKeyHandler` blocked the literal request-body keys `__proto__`, `constructor`, and `prototype` (added in 3.9.3, see GHSA-5fgg-jcpf-8jjw), but did not reject dotted variants such as `"__proto__.polluted"`. Downstream backends that split the missing-key string on a configured `keySeparator` (notably `i18next-fs-backend` ≤ 2.6.5) hand these keys to an unguarded `setPath()` walker that writes to `Object.prototype`.

Applications that expose `missingKeyHandler` to untrusted input **AND** use `i18next-fs-backend` ≤ 2.6.5 are directly exploitable for remote prototype pollution. Other downstream backends that split the missing-key string the same way may be similarly affected.

Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks.

### Patches

Fixed in **i18next-http-middleware 3.9.7**. A new `utils.hasUnsafeKeySegment(key, keySeparator)` helper is now used by `missingKeyHandler`; the configured `i18next.options.keySeparator` is honoured (default `.`; `false` disables segment splitting and only the literal-key denylist applies). Legitimate dotted keys (e.g. `"header.title"`) are unaffected.

The root-cause fix has been shipped in `i18next-fs-backend` **2.6.6** — see the companion advisory.

### Workarounds

If users cannot upgrade immediately:

- Do not expose `missingKeyHandler` to untrusted us…