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Critical, CVSS 9.1amended

i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names

CVE-2026-48714GHSA-f49m-vf83-692w

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (i18next-http-middleware 3.9.7), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
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-2026-48714: 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.
Source description

As published by the source; long descriptions are shortened.

Impact i18next-http-middleware ≤ 3.9.6's 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. Patches Fixed in i18next-http-middleware 3.9.7. A new utils.hasUnsafeKeySegment(key, keySeparator) helper is now used…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-f49m-vf83-692w

  2. Revision 2 · amended ·

    Amended from osv record GHSA-f49m-vf83-692w

  3. Revision 1 · created ·

    Published from ghsa record GHSA-f49m-vf83-692w

Agent brief

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# Patch Next advisory brief: CVE-2026-48714

Advisory: i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names
IDs: CVE-2026-48714, GHSA-f49m-vf83-692w
Record: https://patchnext.com/advisories/cve-2026-48714 (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-http-middleware 3.9.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm i18next-http-middleware: affected < 3.9.7; fixed in 3.9.7

## Suggested inspection (read-only)
- `npm ls i18next-http-middleware --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-http-middleware.
- 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-f49m-vf83-692w (GitHub-reviewed advisory GHSA-f49m-vf83-692w lists affected ranges: npm i18next-http-middleware >=0 <3.9.7)
- Verified: https://osv.dev/vulnerability/GHSA-f49m-vf83-692w (OSV record GHSA-f49m-vf83-692w lists affected ranges: npm i18next-http-middleware >=0 <3.9.7 (fixed))

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