i18next-fs-backend vulnerable to prototype pollution via crafted missing-key string

Impact

i18next-fs-backend ≤ 2.6.5, when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input), is vulnerable to prototype pollution via crafted missing-key strings.

Backend.writeFile() splits each queued missing-key string on the configured keySeparator (default .) before calling the internal setPath() walker. The walker (getLastOfPath in lib/utils.js) did not guard against unsafe segments, so a key like "__proto__.polluted" was split into ["__proto__", "polluted"] and walked straight into Object.prototype, allowing an attacker to write arbitrary properties onto the global object prototype.

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

Affected configuration

Applications are directly affected only if all of the following hold:

• i18next-fs-backend ≤ 2.6.5 is configured as the backend.
• i18next-http-middleware's missingKeyHandler (or another route that forwards untrusted request bodies to i18next.t(..., { ... }) with saveMissing: true) is reachable by untrusted users.
• The default behaviour of splitting missing-key strings on keySeparator is in use (i.e. keySeparator is not false).

Apps that do not expose missing-key persistence to untrusted input are not directly affected through this attack path.

Patches

Fixed in i18next-fs-backend 2.6.6. The traversal helper now refuses to descend through __proto__, constructor, or prototype segments and drops the offending write silently. Legitimate dotted keys (e.g. "header.title") are unaffected.

A matching defence-in-depth fix has been shipped in i18next-http-middleware 3.9.7 — see the companion advisory.

Workarounds

If users cannot upgrade immediately:

• Do not expose i18next-http-middleware's missingKeyHandler to untrusted users (mount it behind authentication, or remove the route).
• Disable missing-key persistence (saveMissing: false, or no backend.create implementation) when accepting writes from untrusted input.
• Set keySeparator: false in the i18next options to disable backend key splitting (note: this also disables nested translation keys).

Resources

• Original report by @codeswhite.
• Companion advisory in i18next-http-middleware: GHSA-f49m-vf83-692w.
• Previous i18next-fs-backend security release: GHSA-8847-338w-5hcj (path traversal via lng/ns, fixed in 2.6.4).