GHSA-F49M-VF83-692W
Vulnerability from github – Published: 2026-06-25 17:28 – Updated: 2026-06-25 17:28Impact
i18next-http-middleware ≤ 3.9.6's missingKeyHandler blocked the literal request-body keys __proto__, constructor, and prototype (added in 3.9.3, see GHSA-5fgg-jcpf-8jjw), but did not reject dotted variants such as "__proto__.polluted". Downstream backends that split the missing-key string on a configured keySeparator (notably i18next-fs-backend ≤ 2.6.5) hand these keys to an unguarded setPath() walker that writes to Object.prototype.
Applications that expose missingKeyHandler to untrusted input AND use i18next-fs-backend ≤ 2.6.5 are directly exploitable for remote prototype pollution. Other downstream backends that split the missing-key string the same way may be similarly affected.
Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks.
Patches
Fixed in i18next-http-middleware 3.9.7. A new utils.hasUnsafeKeySegment(key, keySeparator) helper is now used by missingKeyHandler; the configured i18next.options.keySeparator is honoured (default .; false disables segment splitting and only the literal-key denylist applies). Legitimate dotted keys (e.g. "header.title") are unaffected.
The root-cause fix has been shipped in i18next-fs-backend 2.6.6 — see the companion advisory.
Workarounds
If users cannot upgrade immediately:
- Do not expose
missingKeyHandlerto untrusted users (mount it behind authentication, or remove the route). - Add a request-body filter ahead of the handler that rejects any top-level key containing
__proto__,constructor, orprototypeafter splitting on a configuredkeySeparator. - Disable missing-key persistence (
saveMissing: false) when accepting writes from untrusted input.
Resources
- Original report by @codeswhite.
- Companion advisory in
i18next-fs-backend: GHSA-2933-q333-qg83. - Previous
i18next-http-middlewaresecurity release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "i18next-http-middleware"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.9.7"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-48714"
],
"database_specific": {
"cwe_ids": [
"CWE-1321"
],
"github_reviewed": true,
"github_reviewed_at": "2026-06-25T17:28:12Z",
"nvd_published_at": "2026-06-15T22:16:17Z",
"severity": "CRITICAL"
},
"details": "### Impact\n\n`i18next-http-middleware` \u2264 3.9.6\u0027s `missingKeyHandler` blocked the literal request-body keys `__proto__`, `constructor`, and `prototype` (added in 3.9.3, see GHSA-5fgg-jcpf-8jjw), but did not reject dotted variants such as `\"__proto__.polluted\"`. Downstream backends that split the missing-key string on a configured `keySeparator` (notably `i18next-fs-backend` \u2264 2.6.5) hand these keys to an unguarded `setPath()` walker that writes to `Object.prototype`.\n\nApplications that expose `missingKeyHandler` to untrusted input **AND** use `i18next-fs-backend` \u2264 2.6.5 are directly exploitable for remote prototype pollution. Other downstream backends that split the missing-key string the same way may be similarly affected.\n\nDepending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks.\n\n### Patches\n\nFixed in **i18next-http-middleware 3.9.7**. A new `utils.hasUnsafeKeySegment(key, keySeparator)` helper is now used by `missingKeyHandler`; the configured `i18next.options.keySeparator` is honoured (default `.`; `false` disables segment splitting and only the literal-key denylist applies). Legitimate dotted keys (e.g. `\"header.title\"`) are unaffected.\n\nThe root-cause fix has been shipped in `i18next-fs-backend` **2.6.6** \u2014 see the companion advisory.\n\n### Workarounds\n\nIf users cannot upgrade immediately:\n\n- Do not expose `missingKeyHandler` to untrusted users (mount it behind authentication, or remove the route).\n- Add a request-body filter ahead of the handler that rejects any top-level key containing `__proto__`, `constructor`, or `prototype` after splitting on a configured `keySeparator`.\n- Disable missing-key persistence (`saveMissing: false`) when accepting writes from untrusted input.\n\n### Resources\n\n- Original report by [@codeswhite](https://github.com/codeswhite).\n- Companion advisory in `i18next-fs-backend`: [GHSA-2933-q333-qg83](https://github.com/i18next/i18next-fs-backend/security/advisories/GHSA-2933-q333-qg83).\n- Previous `i18next-http-middleware` security release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).",
"id": "GHSA-f49m-vf83-692w",
"modified": "2026-06-25T17:28:12Z",
"published": "2026-06-25T17:28:12Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48714"
},
{
"type": "WEB",
"url": "https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49"
},
{
"type": "PACKAGE",
"url": "https://github.com/i18next/i18next-http-middleware"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H",
"type": "CVSS_V3"
}
],
"summary": "i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.