i18next i18next-http-middleware up to 3.9.6 setPath prototype pollution

Summaryinfo

A vulnerability has been found in i18next i18next-http-middleware up to 3.9.6 and classified as critical. The affected element is the function setPath. The manipulation leads to prototype pollution. This vulnerability is listed as CVE-2026-48714. The attack may be initiated remotely. There is no available exploit. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in i18next i18next-http-middleware up to 3.9.6 and classified as critical. This issue affects the function setPath. The manipulation with an unknown input leads to a prototype pollution vulnerability. Using CWE to declare the problem leads to CWE-1321. The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Impacted is integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-48714 since 05/22/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1059 according to MITRE ATT&CK.

Upgrading to version 3.9.7 eliminates this vulnerability. Applying the patch 7c6d26f137d3e940b8d229ca148bca38845faf49 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-37006). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Middleware

Vendor

• i18next

Name

• i18next-http-middleware

Version

• 3.9.0
• 3.9.1
• 3.9.2
• 3.9.3
• 3.9.4
• 3.9.5
• 3.9.6

License

• open-source

Website

• Product: https://github.com/i18next/i18next-http-middleware/

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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