---
id: "CVE-2026-63402"
kind: "vulnerability"
aliases: ["CVE-2026-63402"]
title: "i18next: deep resource merge lets attacker corrupt JavaScript built-ins"
severity: "medium"
cvss: 5.9
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["browser", "i18next", "javascript", "node", "package"]
not_affected_if: ["default-config", "no-untrusted-resource-data", "overwrite-off"]
published_at: "2026-10-30T23:17:15+00:00"
first_seen_at: "2026-10-30T23:26:12+00:00"
updated_at: "2026-10-30T23:28:10+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63402", "first_seen_at": "2026-10-30T23:26:12+00:00"}]
---

# i18next: deep resource merge lets attacker corrupt JavaScript built-ins

i18next's deepExtend, reached through addResourceBundle with deep and overwrite both true, can follow inherited Object.prototype built-ins such as hasOwnProperty, so attacker-supplied resource data overwrites them process-wide. The result is denial of service through TypeErrors across the application, with limited integrity impact.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `npm ls i18next` to find the installed version and whether it is below `26.3.4`.
- Search the codebase for calls to `addResourceBundle(` that pass `deep` and `overwrite` as `true` with data that comes from users or external input.

## Fix

- i18next: move to `26.3.4` or later.

## Timeline

- 2026-10-30T23:26:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-63402

## From the sources

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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_