Published: October 30, 2026 | Last Modified: October 30, 2026
i18next is an internationalization framework for JavaScript. Prior to version 26.3.4, `deepExtend` in `src/utils.js` checks property existence with the in operator and recurses into inherited built-ins reached through `Object.prototype`. An application must explicitly pass attacker-controlled resource data to `addResourceBundle(lng, ns, data, deep, overwrite)` with both `deep: true` and `overwrite: true`. Source keys such as `hasOwnProperty`, `toString` and `valueOf` can then select shared inherited built-in function objects, and a nested value can overwrite a property of those objects process-wide. For example, making `Object.prototype.hasOwnProperty.call` non-callable causes subsequent uses, including i18next internals, to throw TypeError, resulting in unrecoverable denial of service and limited integrity impact. The `__proto__`/`constructor` guard does not prevent this mechanism because the corruption target is an inherited built-in function rather than the prototype object itself. Default calls without deep merging or with overwrite falsy are unaffected, and no standard i18next backend or integration uses the vulnerable configuration. This issue is distinct from the setPath/key-walking prototype-pollution flaws in i18next-fs-backend (CVE-2026-48713) and i18next-http-middleware (CVE-2026-48714). This issue is fixed in version 26.3.4.
Exploitation Probability (EPSS): Low — 0.26% (18th percentile)
Low exploitation probability based on current threat landscape data. Standard patching timeline is appropriate.
This analysis is generated by Ghostwire from NVD, CISA KEV, EPSS, and open-source intelligence data. Verify findings through primary sources before acting.