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).

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - 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).
    (CVE-2026-48713)

ID	Name	Product	Family	Type	Severity
443972	SCA: security update for i18next-fs-backend (GHSA-2933-q333-qg83)	Tenable Cloud Security	SCA Checks	Local	critical
443972	SCA: security update for i18next-fs-backend (GHSA-2933-q333-qg83)	Tenable Self-Hosted Container Security	SCA Checks	Local	critical

Detections

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CVE-2026-48713

critical

Tenable Plugins

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critical