i18next-http-middleware missingKeyHandler does not reject keys whose segments contain prototype-polluting names_CVE-2026-48714

{
    "lastseen": "",
    "description": "i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. In versions prior to 3.9.7, the 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. Applications 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. Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks. This issue has been fixed in version 3.9.7.  If developers cannot upgrade immediately, they should do the following: do not expose missingKeyHandler to 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, or prototype after splitting on their configured keySeparator, and disable missing-key persistence (saveMissing: false) when accepting writes from untrusted input.",
    "published": "2026-06-15T20:41:49.382Z",
    "modified": "2026-06-15T20:41:49.382Z",
    "type": "cve",
    "title": "i18next-http-middleware missingKeyHandler does not reject keys whose segments contain prototype-polluting names",
    "source": "GitHub_M",
    "references": "https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w\nhttps://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49",
    "id": "CVE-2026-48714",
    "bulletinFamily": "",
    "cwe": [
        "CWE-1321"
    ],
    "cvelist": null,
    "sourceData": "i18next i18next-http-middleware < 3.9.7",
    "sourceHref": "",
    "cvss": {
        "score": 9.1,
        "severity": "CRITICAL",
        "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H",
        "version": "3.1"
    },
    "cvss2": [],
    "cvss3": {
        "version": "",
        "vectorString": "",
        "baseScore": 0,
        "baseSeverity": "",
        "attackVector": "",
        "attackComplexity": "",
        "privilegesRequired": "",
        "userInteraction": "",
        "scope": "",
        "confidentialityImpact": "",
        "integrityImpact": "",
        "availabilityImpact": "",
        "cvssV3": {
            "version": "",
            "vectorString": "",
            "baseScore": 0,
            "baseSeverity": "",
            "attackVector": "",
            "attackComplexity": "",
            "privilegesRequired": "",
            "userInteraction": "",
            "scope": "",
            "confidentialityImpact": "",
            "integrityImpact": "",
            "availabilityImpact": ""
        }
    },
    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "i18next-http-middleware",
    "version": "< 3.9.7",
    "vendor": "i18next",
    "ai_description": "Remote prototype pollution vulnerability in i18next-http-middleware due to missingKeyHandler not rejecting keys with prototype-polluting names",
    "ai_severity": "Critical",
    "ai_vendor": "i18next",
    "ai_product": "i18next-http-middleware",
    "ai_version": "< 3.9.7",
    "ai_score": 9.1
}