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 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.

CVSS v3 5.9 MEDIUM

5.9^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 1.6 / Impact : 4.2

Attack Vector NETWORK

Attack Complexity HIGH

Privileges Required LOW

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact LOW

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://github.com/i18next/i18next/commit/642137b7786d83661ec3ee53027798dc4b81d30a
https://github.com/i18next/i18next/security/advisories/GHSA-6jcc-5g8w-32mx
https://github.com/i18next/i18next/tree/v26.3.4

Configurations

No configuration.

History

30 Oct 2026, 23:17

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-10-30 23:17

Updated : 2026-10-30 23:17

NVD link : CVE-2026-63402

Mitre link : CVE-2026-63402

CVE.ORG link : CVE-2026-63402

JSON object : View

Products Affected

No product.

CWE

CWE-471

Modification of Assumed-Immutable Data (MAID)

CWE-1321

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

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