i18next-fs-backend vulnerable to prototype pollution via crafted missing-key string

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

9.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

EPSS

• CVE-2026-48713: 0.49% estimated exploitation activity in the next 30 days; percentile 42.33 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact i18next-fs-backend ≤ 2.6.5, when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input), is vulnerable to prototype pollution via crafted missing-key strings. 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. Affected configuration Applications are directly affected only if all of the following hold: - i18next-fs-backend ≤ 2.6.5 is configured as the…

Published by the source

25 Jun 2026, 17:28 UTC

Last modified by the source

25 Jun 2026, 17:56 UTC

Source last checked

26 Oct 2026, 12:56 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-2933-q333-qg83

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-2933-q333-qg83

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-2933-q333-qg83

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# Patch Next advisory brief: CVE-2026-48713

Advisory: i18next-fs-backend vulnerable to prototype pollution via crafted missing-key string
IDs: CVE-2026-48713, GHSA-2933-q333-qg83
Record: https://patchnext.com/advisories/cve-2026-48713 (revision 3, source checked 26 Oct 2026, 12:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: i18next-fs-backend 2.6.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm i18next-fs-backend: affected < 2.6.6; fixed in 2.6.6

## Suggested inspection (read-only)
- `npm ls i18next-fs-backend --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on i18next-fs-backend.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2933-q333-qg83 (GitHub-reviewed advisory GHSA-2933-q333-qg83 lists affected ranges: npm i18next-fs-backend >=0 <2.6.6)
- Verified: https://osv.dev/vulnerability/GHSA-2933-q333-qg83 (OSV record GHSA-2933-q333-qg83 lists affected ranges: npm i18next-fs-backend >=0 <2.6.6 (fixed))

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