i18next-http-middleware has path traversal / SSRF via user-controlled language and namespace parameters

Summary

Versions of i18next-http-middleware prior to 3.9.3 pass the user-controlled lng and ns values from getResourcesHandler directly into i18next.services.backendConnector.load(languages, namespaces, …) without any sanitisation. Depending on which backend is configured, the unvalidated path segments enable one of two attacks:

• Filesystem path traversal when the middleware is paired with i18next-fs-backend (or any backend that interpolates lng / ns into a filesystem path).
• Server-Side Request Forgery (SSRF) when the middleware is paired with i18next-http-backend (or any backend that interpolates into an HTTP URL).

Example request:

GET /locales/resources.json?lng=../../etc/passwd&ns=root

with i18next-fs-backend reads the attacker-chosen file from disk; with i18next-http-backend reshapes the outgoing URL to target an internal service.

Impact

• Arbitrary file read via fs-style backends — any file the Node process can read becomes reachable (source, configuration, .ssh keys, .env, Docker secrets, etc.).
• SSRF via http-style backends — requests to internal IPs / hostnames not normally reachable from the internet; combined with cloud metadata endpoints this can escalate to credential theft.
• Unbounded growth of i18next.options.ns — a now-incidental amplification: the pre-patch getResourcesHandler pushed every unique ns value into the shared i18next.options.ns singleton array without validation or bounds, enabling memory exhaustion from repeated unique payloads.

The severity is bounded by the backend in place, but the middleware itself exposed the unsanitised path; this is the "weakest link" layer.

Affected versions

< 3.9.3.

Patch

Fixed in 3.9.3. The patch introduces utils.isSafeIdentifier and applies it in getResourcesHandler before lng and ns reach the backend connector:

languages  = languages.filter(utils.isSafeIdentifier)
namespaces = namespaces.filter(utils.isSafeIdentifier)

isSafeIdentifier uses a denylist approach — it still accepts any legitimate i18next language-code shape (i18next FAQ) — rejecting:

• .. sequences (relative path traversal)
• path separators (/, \)
• control characters (C0/C1)
• prototype keys (__proto__ / constructor / prototype)
• empty strings and values longer than 128 characters

Unsafe values are dropped; only safe values reach the backend. The fix is a defence-in-depth layer on top of any sanitisation the backend itself may apply.

Workarounds

No workaround short of upgrading. Front-proxying the middleware with a WAF rule that rejects requests containing .., /, \, or URL-structure characters in lng / ns is a partial mitigation. Upgrading the configured backend (i18next-fs-backend ≥ 2.6.4, i18next-http-backend ≥ 3.0.5) also closes the same attack at the next layer.

Related advisories fixed in the same release

• GHSA-5fgg-jcpf-8jjw — prototype pollution via setPath and missingKeyHandler. Independently fixable, filed separately per CNA rules.
• GHSA-c3h8-g69v-pjrg — HTTP response splitting + XSS-filter bypass (CVE-2026-41683).

Credits

Discovered via an internal security audit of the i18next ecosystem.

Resources

• CWE-22: Path Traversal
• CWE-918: Server-Side Request Forgery (SSRF) (specific sub-case when paired with an HTTP backend)
• i18next FAQ: language code formatting