Vulnerability-Lookup

CVE-2026-48714 (GCVE-0-2026-48714)

Vulnerability from cvelistv5 – Published: 2026-06-15 20:41 – Updated: 2026-06-16 14:50

Title

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

Summary

i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. In versions prior to 3.9.7, the 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. This issue has been fixed in version 3.9.7. If developers cannot upgrade immediately, they should do the following: do not expose missingKeyHandler to untrusted users (mount it behind authentication, or remove the route), add a request-body filter ahead of the handler that rejects any top-level key containing __proto__, constructor, or prototype after splitting on their configured keySeparator, and disable missing-key persistence (saveMissing: false) when accepting writes from untrusted input.

Severity

9.1 (Critical)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-06-16 14:50 UTC

CWE

CWE-1321 - Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Assigner

GitHub_M mitre root

References

2 references

URL	Tags
https://github.com/i18next/i18next-http-middlewar…	x_refsource_CONFIRM
https://github.com/i18next/i18next-http-middlewar…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
i18next	i18next-http-middleware	Affected: < 3.9.7	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-48714

Vulnerability from fkie_nvd - Published: 2026-06-15 22:16 - Updated: 2026-06-18 15:21

Severity

9.1 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49	Patch
	security-advisories@github.com	https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w	Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	i18next	i18next-http-middleware	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "i18next-http-middleware",
          "vendor": "i18next",
          "versions": [
            {
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            }
          ]
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      ],
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    }
  ],
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GHSA-F49M-VF83-692W

Vulnerability from github – Published: 2026-06-25 17:28 – Updated: 2026-06-25 17:28

Summary

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

Details

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 by missingKeyHandler; the configured i18next.options.keySeparator is honoured (default .; false disables segment splitting and only the literal-key denylist applies). Legitimate dotted keys (e.g. "header.title") are unaffected.

The root-cause fix has been shipped in i18next-fs-backend 2.6.6 — see the companion advisory.

Workarounds

If users cannot upgrade immediately:

Do not expose missingKeyHandler to untrusted users (mount it behind authentication, or remove the route).
Add a request-body filter ahead of the handler that rejects any top-level key containing __proto__, constructor, or prototype after splitting on a configured keySeparator.
Disable missing-key persistence (saveMissing: false) when accepting writes from untrusted input.

Resources

Original report by @codeswhite.
Companion advisory in i18next-fs-backend: GHSA-2933-q333-qg83.
Previous i18next-http-middleware security release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).

Severity

9.1 (Critical)


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

Show details on source website

JSON

To clipboard

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    {
      "package": {
        "ecosystem": "npm",
        "name": "i18next-http-middleware"
      },
      "ranges": [
        {
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            {
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            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48714"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-25T17:28:12Z",
    "nvd_published_at": "2026-06-15T22:16:17Z",
    "severity": "CRITICAL"
  },
  "details": "### Impact\n\n`i18next-http-middleware` \u2264 3.9.6\u0027s `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` \u2264 2.6.5) hand these keys to an unguarded `setPath()` walker that writes to `Object.prototype`.\n\nApplications that expose `missingKeyHandler` to untrusted input **AND** use `i18next-fs-backend` \u2264 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.\n\nDepending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks.\n\n### Patches\n\nFixed in **i18next-http-middleware 3.9.7**. A new `utils.hasUnsafeKeySegment(key, keySeparator)` helper is now used by `missingKeyHandler`; the configured `i18next.options.keySeparator` is honoured (default `.`; `false` disables segment splitting and only the literal-key denylist applies). Legitimate dotted keys (e.g. `\"header.title\"`) are unaffected.\n\nThe root-cause fix has been shipped in `i18next-fs-backend` **2.6.6** \u2014 see the companion advisory.\n\n### Workarounds\n\nIf users cannot upgrade immediately:\n\n- Do not expose `missingKeyHandler` to untrusted users (mount it behind authentication, or remove the route).\n- Add a request-body filter ahead of the handler that rejects any top-level key containing `__proto__`, `constructor`, or `prototype` after splitting on a configured `keySeparator`.\n- Disable missing-key persistence (`saveMissing: false`) when accepting writes from untrusted input.\n\n### Resources\n\n- Original report by [@codeswhite](https://github.com/codeswhite).\n- Companion advisory in `i18next-fs-backend`: [GHSA-2933-q333-qg83](https://github.com/i18next/i18next-fs-backend/security/advisories/GHSA-2933-q333-qg83).\n- Previous `i18next-http-middleware` security release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).",
  "id": "GHSA-f49m-vf83-692w",
  "modified": "2026-06-25T17:28:12Z",
  "published": "2026-06-25T17:28:12Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48714"
    },
    {
      "type": "WEB",
      "url": "https://github.com/i18next/i18next-http-middleware/commit/7c6d26f137d3e940b8d229ca148bca38845faf49"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/i18next/i18next-http-middleware"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names"
}

Sightings

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Action not permitted

CVE-2026-48714 (GCVE-0-2026-48714)

FKIE_CVE-2026-48714

GHSA-F49M-VF83-692W

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