i18next-fs-backend: Prototype pollution via crafted missing-key string

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

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).

Source published

15 Jun 2026 20:31:53 UTC

Source modified

16 Jun 2026 12:49:24 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

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).

Source published

Jun 15, 2026, 8:31:53 PM (timezone not stated)

Source modified

Jun 16, 2026, 12:49:24 PM (timezone not stated)

Observed here

7 Nov 2026 11:30:20 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

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).

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-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 backend. - `i18next-http-middleware`'s `missingKeyHandler` (or another route that forwards untrusted request bodies to `i18next.t(..., { ... })` with `saveMissing: true`) is reachable by untrusted users. - 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. ### Patches Fixed in **i18next-fs-backend 2.6.6**. The traversal helper now refuses to descend through `__proto__`, `constructor`, or `prototype` segments and drops the offending write silently. Legitimate dotted keys (e.g. `"header.title"`) are unaffected. A matching defence-in-depth fix has been shipped in `i18next-http-middleware` **3.9.7** — see the companion advisory. ### Workarounds If users cannot upgrade immediately: - 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. - Set `keySeparator: false` in the i18next options to disable backend key splitting (note: this also disables nested translation keys). ### Resources - Original report by [@codeswhite](https://github.com/codeswhite). - Companion advisory in `i18next-http-middleware`: [GHSA-f49m-vf83-692w](https://github.com/i18next/i18next-http-middleware/security/advisories/GHSA-f49m-vf83-692w). - Previous `i18next-fs-backend` security release: GHSA-8847-338w-5hcj (path traversal via `lng`/`ns`, fixed in 2.6.4).

Source published

25 Jun 2026 17:28:46 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

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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: 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