5.9 Medium

i18next is an internationalization framework for JavaScript.

CVE-2026-63402

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…

Assessment

CVSS: 5.9 (v3.1)
Severity: Medium
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 30 October 2026
Last updated: 30 October 2026
Affected: i18next
Assigner: GitHub_M

CVSS vectorCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H

At the source

EUVD (ENISA)

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