{"id":"CVE-2026-48714","aliases":["EUVD-2026-37006","GHSA-f49m-vf83-692w"],"status":"published","title":"i18next-http-middleware missingKeyHandler does not reject keys whose segments contain prototype-polluting names","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 ≤ 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. 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.","severity":{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00492,"percentile":0.42618,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1321"],"published":"2026-06-15T20:41:49Z","modified":"2026-10-28T02:30:43Z","fixes":[{"ecosystem":"npm","package":"i18next-http-middleware","fixed_in":["3.9.7"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"i18next-http-middleware","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.9.7"}]}]},{"source":"osv","ecosystem":"npm","package":"i18next-http-middleware","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"3.9.7"}]}]}],"products":[{"vendor":"i18next","product":"i18next-http-middleware"}],"references":[{"url":"https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w","tags":["x_refsource_CONFIRM","Mitigation","Vendor Advisory","advisory","web"]},{"url":"https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49","tags":["x_refsource_MISC","Patch","fix","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-48714","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-f49m-vf83-692w"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48714.json","tags":["advisory"]},{"url":"https://github.com/i18next/i18next-http-middleware","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-48714","title":"i18next-http-middleware missingKeyHandler does not reject keys whose segments contain prototype-polluting names","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 ≤ 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. 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.","status":"published","published":"2026-06-15T20:41:49.382Z","modified":"2026-06-16T14:50:29.087Z","cvss":[{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical","assessor":"cna"}],"cwes":["CWE-1321"],"updated_at":"2026-11-02T13:50:49.872037Z"}],"euvd":[{"id":"EUVD-2026-37006","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 ≤ 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. 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.","status":"published","published":"2026-06-15T20:41:49Z","modified":"2026-06-16T14:50:29Z","cvss":[{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical"}],"updated_at":"2026-11-02T18:20:40.605509Z"}],"ghsa":[{"id":"GHSA-f49m-vf83-692w","title":"i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names","description":"### Impact\n\n`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`.\n\nApplications 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.\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** — 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).","status":"published","published":"2026-06-25T17:28:12Z","modified":"2026-06-25T17:28:12Z","cvss":[{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical"}],"cwes":["CWE-1321"],"updated_at":"2026-11-02T14:40:33.778544Z"}],"nvd":[{"id":"CVE-2026-48714","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 ≤ 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. 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.","status":"published","published":"2026-06-15T22:16:17.55Z","modified":"2026-10-25T23:10:00.167Z","cvss":[{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical","assessor":"secondary"}],"cwes":["CWE-1321"],"updated_at":"2026-11-02T14:12:24.19758Z"}],"osv":[{"id":"CVE-2026-48714","title":"i18next-http-middleware missingKeyHandler does not reject keys whose segments contain prototype-polluting names","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 ≤ 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. 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.","status":"published","published":"2026-06-15T20:41:49.382Z","modified":"2026-10-28T02:30:43.157224202Z","cvss":[{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical"}],"cwes":["CWE-1321"],"updated_at":"2026-11-02T15:45:06.701612Z"},{"id":"GHSA-f49m-vf83-692w","title":"i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names","description":"### Impact\n\n`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`.\n\nApplications 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.\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** — 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).","status":"published","published":"2026-06-25T17:28:12Z","modified":"2026-06-25T17:56:29.524803Z","cvss":[{"version":"3.1","score":9.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","level":"critical"}],"cwes":["CWE-1321"],"updated_at":"2026-11-02T15:46:46.006631Z"}]}}