CVE-2026-48713
i18next-fs-backend: Prototype pollution via crafted missing-key string
- Published
- Updated
- Reserved
- Assigned by
- GitHub_M
Description
Versions prior to 2.6.6 are vulnerable to prototype pollution via crafted missing-key strings when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input). Backend.writeFile() splits each queued missing-key string on the configured keySeparator (default .) before calling the internal setPath() walker. The walker (getLastOfPath in lib/utils.js) did not guard against unsafe segments, so a key like "__proto__.polluted" was split into ["__proto__", "polluted"] and walked straight into Object.prototype, allowing an attacker to write arbitrary properties onto the global object prototype. Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks. Applications are affected only if the missingKeyHandler (or another route that forwards untrusted request bodies to i18next.t(..., { ... }) with saveMissing: true) is reachable by untrusted users and the default behaviour of splitting missing-key strings on keySeparator is in use (i.e. keySeparator is not false). Apps that do not expose missing-key persistence to untrusted input are not directly affected through this attack path. This issue has been fixed in version 2.6.6. If developers using the library are unable to upgrade immediately, they should take the following precautions: do not expose i18next-http-middleware's missingKeyHandler to untrusted users (mount it behind authentication, or remove the route), disable missing-key persistence (saveMissing: false, or no backend.create implementation) when accepting writes from untrusted input, and set keySeparator: false in their i18next options to disable backend key splitting (note: this also disables nested translation keys).
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_MCNA (assigner) | 3.1 | Critical9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H |
| NVDNVD · Analyzed | 3.1 | Critical9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H |
- EPSS (exploitation in 30 days)
- 0.49% · top 58% of all CVEs (6 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: yes · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: Critical 9.1.
Weakness
Affected products
- affected < 2.6.6
Affected packages
From reviewed advisories of the GitHub Advisory Database.
- npmi18next-fs-backendGHSA-2933-q333-qg83 (opens in a new tab)
References (2)
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisory GHSA-2933-q333-qg83 (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://trackcve.app/cve/CVE-2026-48713)Record last updated .