i18next: Shared built-in corruption through inherited-key deepExtend recursion
Only affects i18next up to 26.3.3 when apps pass attacker-controlled data to addResourceBundle with deep and overwrite both set to true. Fixed in 26.3.4.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Fix
- Fixed in 26.3.4
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
i18next, a JavaScript internationalization framework, is affected when an application passes untrusted resource data to its deep-merge path with overwrite enabled. This can corrupt shared built-in functions across the whole process, causing TypeErrors and an unrecoverable denial of service with limited integrity impact.
Summarized by anthropic/claude-haiku-5.5. Verify details against the original sources.
Read the original description
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.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- i18next i18next
fixed in 26.3.4
References 3
GitHub · i18next/i18next
Questions about CVE-2026-63402
What does this mean?
A crafted key can make the deep-merge logic overwrite shared built-in Object.prototype functions. Once those are replaced, code across the whole process starts throwing TypeErrors, and the process cannot recover, which is a process-wide denial of service with limited integrity impact.Source: github.com
Am I affected, and how do I check?
Versions up to and including 26.3.3 are affected, but only when the application passes attacker-controlled resource data into addResourceBundle with both deep: true and overwrite: true. Check the installed i18next version and whether calls to addResourceBundle use those two options with data that comes from outside the application.Source: github.com
What should I do about it?
Upgrade i18next to 26.3.4, which fixes the issue by checking property existence with Object.prototype.hasOwnProperty.call(target, prop).Source: github.com
Is it being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data