CVE-2026-48714: 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…

PriorityP258critical9.1CVSS 3.1

AVNACLPRNUINSUCNIHAH

EPSS

0.42%

33.5th percentile

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.

Affected

2 ranges

Vendor	Product	Version range	Fixed in
i18next	i18next-http-middleware	< 3.9.7	3.9.7
i18next	i18next-http-middleware	>= 0 < 3.9.7	3.9.7

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