{"id":"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.","aliases":["GHSA-f49m-vf83-692w"],"modified":"2026-08-05T03:32:17.470561251Z","published":"2026-06-15T20:41:49.382Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48714.json","unresolved_ranges":[{"extracted_events":[{"fixed":"3.9.7"}],"source":"AFFECTED_FIELD"}],"cna_assigner":"GitHub_M","cwe_ids":["CWE-1321"]},"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"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/i18next/i18next-http-middleware","events":[{"introduced":"0"},{"fixed":"7c6d26f137d3e940b8d229ca148bca38845faf49"}],"database_specific":{"source":"REFERENCES"}}],"versions":["v3.9.6","v3.9.5","v3.9.4","v3.9.3","v3.9.2","v3.9.1","v3.9.0","v3.8.3","v3.8.2","v3.8.1","v3.8.0","v3.7.4","v3.7.3","v3.7.2","v3.7.1","v3.7.0","v3.6.0","v3.5.0","v3.4.1","v3.4.0","v3.3.2","v3.3.1","v3.3.0","v3.2.2","v3.2.1","v3.2.0","v3.1.6","v3.1.5","v3.1.4","v3.1.3","v3.1.2","v3.1.1","v3.1.0","v3.0.6","v3.0.5","v3.0.4","v3.0.3","v3.0.2","v3.0.1","v3.0.0","v2.1.2","v2.1.1","v2.1.0","v2.0.0","v1.3.1","v1.3.0","v1.2.1","v1.2.0","v1.1.0","v1.0.7","v1.0.6","v1.0.5","v1.0.4","v1.0.3","v1.0.2","v1.0.1","v1.0.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-48714.json"}}],"schema_version":"1.8.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H"}]}