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VulnerabilityPublished

CVE-2026-48713: i18next-fs-backend · Prototype Pollution

Affects i18next-fs-backendSource CVE Database V5

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Not known to be exploited · CVSS 9.1 Critical : upgrade to 2.6.6 as a priority.

Data as of 1 Nov 2026, 22:58 UTC

CVSS 3.1 base score
9.1Critical
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.4% chance of exploitation in 30 days – higher than 42% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 2.6.6
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 15 Jun 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 1 Nov 2026.

What to do

Upgrade i18next-fs-backend and i18next/i18next-fs-backend to 2.6.6.

Affected software

ProductAffectedFixed in
i18nexti18next-fs-backendAffected
  • <2.6.6
Fixed in
  • 2.6.6
npmi18next/i18next-fs-backendpkg:npm/i18next/i18next-fs-backendOther threatsAffected
  • <2.6.6
Fixed in
  • 2.6.6

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available in i18next-fs-backend version 2.6.6. Users should upgrade to this version to remediate the vulnerability. If immediate upgrade is not possible, mitigate by ensuring the missingKeyHandler or equivalent routes are not exposed to untrusted users (e.g., restrict access via authentication), disable missing-key persistence by setting saveMissing to false or removing backend.create implementations when accepting untrusted input, and set keySeparator to false to disable backend key splitting (noting this disables nested translation keys).

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).

References

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