Prototype Pollution in i18next-fs-backend missing-key persistence
CVE-2026-48713 is a prototype pollution vulnerability in i18next-fs-backend affecting versions prior to 2.6.6. The flaw is in the missing-translation-key persistence path: Backend.writeFile() splits queued missing-key strings on the configured keySeparator, which defaults to '.', and then passes the resulting path segments to the internal path walker/setter logic, including getLastOfPath in lib/utils.js. Unsafe path segments such as 'proto', 'constructor', or 'prototype' were not blocked. As a result, a crafted missing key such as 'proto.polluted' could be split into ['proto','polluted'] and traversed into Object.prototype, allowing attacker-controlled properties to be written onto the global object prototype. The vulnerable path is reachable when applications persist missing translation keys from untrusted input, such as through i18next-http-middleware's missingKeyHandler or similar routes forwarding untrusted request bodies to i18next.t(..., { ... }) with saveMissing enabled.
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A critical prototype pollution vulnerability in i18next-fs-backend that allows an unauthenticated attacker to write arbitrary properties onto Object.prototype via crafted translation keys when missing-key persistence is exposed to untrusted input.
A critical prototype pollution vulnerability in i18next-fs-backend caused by unsafe handling of crafted missing-key strings, allowing writes to Object.prototype when missing-key persistence is exposed to untrusted input.
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