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

> Attackers can achieve remote prototype pollution via crafted keys in missingKeyHandler when using vulnerable i18next-fs-backend, potentially leading to code execution.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Jun 2026
- **Updated:** 16 Jun 2026
- **Also known as:** GHSA-f49m-vf83-692w, MINI-vwr9-jcqh-qvcf

## What is CVE-2026-48714?

CVE-2026-48714 is a critical unsafe deserialization vulnerability in i18next-http-middleware, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-48714?

2 open source packages ship an affected version, including i18next-http-middleware. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `i18next-http-middleware` | npm | All versions | 3.9.7 |
| `i18next-http-middleware` | npm | All versions | 3.9.7 |

Products: I18next I18next-Http-Middleware.

## How do I fix CVE-2026-48714?

Upgrade i18next-http-middleware to 3.9.7 or later.

## Is CVE-2026-48714 being exploited?

There is no known exploitation of CVE-2026-48714 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-48714 and fix it if it is affected.

What it is: CVE-2026-48714 is a critical unsafe deserialization vulnerability in i18next-http-middleware, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-48714 in the wild and no public exploit code.

Affected packages and fixed versions:
- i18next-http-middleware (npm): affected all versions; fixed in 3.9.7
- i18next-http-middleware (npm): affected all versions; fixed in 3.9.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-48714/ (Markdown: https://insights.sbomify.com/CVE-2026-48714/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-48714
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-48714
- OSV: https://osv.dev/vulnerability/CVE-2026-48714

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-48714/
Summaries are generated and may contain mistakes; the official description above is the record.