Executive brief
i18next-fs-backend is a Node.js library that handles translation file management for the i18next internationalization framework. When configured to save missing translation keys (especially via i18next-http-middleware), the library fails to sanitize user-supplied key names before storing them. An attacker can inject specially crafted key strings to pollute the JavaScript object prototype, causing application misbehavior, crashes, or bypassing security checks—similar to injecting code into a system's core configuration layer.
Technical details
The vulnerability is a prototype pollution flaw (CWE-1321) in the i18next-fs-backend library, versions ≤ 2.6.5. The Backend.writeFile() method splits missing-key strings on the configured keySeparator (default ".") before passing them to an internal setPath() traversal helper. The getLastOfPath() function in lib/utils.js fails to block traversal through dangerous prototype property names like "__proto__", "constructor", and "prototype". An attacker can reach this code by supplying crafted key names to endpoints like i18next-http-middleware's missingKeyHandler when saveMissing is enabled, if that handler is exposed to untrusted input. By submitting a key such as "__proto__.polluted", the attacker causes the walker to traverse into Object.prototype and set arbitrary global properties. The fix in version 2.6.6 guards the traversal helper to refuse descending through unsafe segments and silently drops offending writes. A defense-in-depth patch was also released in i18next-http-middleware 3.9.7.
Affected products
- i18next i18next-fs-backend < 2.6.6
Timeline
- 2026-05-22: advisory: Coordinated disclosure by @codeswhite
- 2026-05-22: patched: Fixed in i18next-fs-backend 2.6.6
- 2026-06-25: disclosed