---
title: "CVE-2026-48714: i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names"
url: https://erroneo.us/security/cve/CVE-2026-48714/
description: "CVE-2026-48714 (i18next-http-middleware (npm)): i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-06-15
date_modified: 2026-06-25
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-48714"
kev: false
cvss: 9.1
epss: 0.00657
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-48714: i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.7% , percentile 51: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 51% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes i18next-http-middleware (npm) < 3.9.7, fixed in 3.9.7 . Check your lockfile, including transitive dependencies.

## What it is

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 packages

- npm `i18next-http-middleware` < 3.9.7 → fixed: 3.9.7

## Affected products (CVE record)

- i18next i18next-http-middleware: < 3.9.7

## Severity

- GitHub_M CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`
- GitHub CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact partial .
- FIRST EPSS (model of 2026-10-21): 0.66% probability of exploitation activity in the next 30 days; percentile 51.

## Sources

- [GitHub Advisory GHSA-f49m-vf83-692w](https://github.com/advisories/GHSA-f49m-vf83-692w)
- [CVE.org record CVE-2026-48714](https://www.cve.org/CVERecord?id=CVE-2026-48714)
- [NVD entry CVE-2026-48714](https://nvd.nist.gov/vuln/detail/CVE-2026-48714)
- [github.com](https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w)
- [github.com](https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49)

---
Source page: https://erroneo.us/security/cve/CVE-2026-48714/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt