{"id":"CVE-2026-48714","aliases":["GHSA-f49m-vf83-692w"],"url":"https://o3.security/vulnerability/CVE-2026-48714","summary":"i18next-http-middleware missingKeyHandler does not reject keys whose segments contain prototype-polluting names","details":"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.","published":"2026-06-15T20:41:49.382Z","modified":"2026-08-12T03:51:13.035459969Z","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"},"epss":{"score":0.00507,"percentile":0.4201,"asOf":"2026-09-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"i18next-http-middleware","fixedVersion":"3.9.7"}],"fix":{"url":"https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49","label":"i18next/i18next-http-middleware@7c6d26f"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48714.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:13.035459969Z"}}