CVE-2026-48713 i18next-fs-backend: Prototype pollution via crafted missing-key string

What is CVE-2026-48713?

CVE-2026-48713 is a critical code injection vulnerability in i18next-fs-backend, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Versions prior to 2.6.6 are vulnerable to prototype pollution via crafted missing-key strings when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input). Backend.writeFile() splits each queued missing-key string on the configured keySeparator (default .) before calling the internal setPath() walker. The walker (getLastOfPath in lib/utils.js) did not guard against unsafe segments, so a key like "__proto__.polluted" was split into ["__proto__", "polluted"] and walked straight into Object.prototype, allowing an attacker to write arbitrary properties onto the global object prototype. Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks. Applications are affected only if the missingKeyHandler (or another route that forwards untrusted request bodies to i18next.t(..., { ... }) with saveMissing: true) is reachable by untrusted users and the default behaviour of splitting missing-key strings on keySeparator is in use (i.e. keySeparator is not false). Apps that do not expose missing-key persistence to untrusted input are not directly affected through this attack path. This issue has been fixed in version 2.6.6. If developers using the library are unable to upgrade immediately, they should take the following precautions: do not expose i18next-http-middleware's missingKeyHandler to untrusted users (mount it behind authentication, or remove the route), disable missing-key persistence (saveMissing: false, or no backend.create implementation) when accepting writes from untrusted input, and set keySeparator: false in their i18next options to disable backend key splitting (note: this also disables nested translation keys).

Weakness: CWE-1321

Which versions are affected by CVE-2026-48713?

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

Products include I18next I18next-Fs-Backend.

How do I fix CVE-2026-48713?

Upgrade i18next-fs-backend to 2.6.6 or later.

Upgrade, and never evaluate templates, expressions or code built from user input. More about Code injection.

Is CVE-2026-48713 being exploited?

There is no known exploitation of CVE-2026-48713 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-2933-q333-qg83, MINI-c3g2-64h6-xx6r, MINI-f58r-w5rv-w66w.