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

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

15 Jun 2026 20:41:49 UTC

Source modified

16 Jun 2026 14:50:29 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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

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.

Source published

Jun 15, 2026, 8:41:49 PM (timezone not stated)

Source modified

Jun 16, 2026, 2:50:29 PM (timezone not stated)

Observed here

7 Nov 2026 11:30:21 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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

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.

Source published

15 Jun 2026 22:16:17 UTC

Source modified

25 Oct 2026 23:10:00 UTC

Observed here

27 Oct 2026 13:11:53 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

### 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](https://github.com/codeswhite). - Companion advisory in `i18next-fs-backend`: [GHSA-2933-q333-qg83](https://github.com/i18next/i18next-fs-backend/security/advisories/GHSA-2933-q333-qg83). - Previous `i18next-http-middleware` security release: GHSA-5fgg-jcpf-8jjw and GHSA-c3h8-g69v-pjrg (in 3.9.3).

Source published

25 Jun 2026 17:28:12 UTC

Source modified

25 Jun 2026 17:56:29 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Publisher rating · CRITICAL