i18next-fs-backend is a backend layer for i18next using in Node.js and for Deno to load translations from the filesystem. Prior to version 2.6.4, i18next-fs-backend substitutes the lng and ns options directly into the configured loadPath / addPath templates and then read / write the resulting file from disk. The interpolation is unencoded and unvalidated, so a crafted lng or ns value — containing .., a path separator, a control character, a prototype key, or simply an unexpectedly long string — allows an attacker who can influence either value to read or overwrite files outside the intended locale directory. When lng / ns are derived from untrusted input (request-scoped i18next instances behind an HTTP layer such as i18next-http-middleware, or any framework that lets the end user pick the language via query string, cookie, or header), a single request such as ?lng=../../../../etc/passwd causes the backend to attempt to read that path. This issue has been patched in version 2.6.4.

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).

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 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 a configured keySeparator. - Disable missing-key persistence (saveMissing: false) when accepting writes from untrusted input.

Resources

- Original report by @codeswhite. - Companion advisory in i18next-fs-backend: GHSA-2933-q333-qg83. - Previous i18next-http-middleware security release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).