Executive brief
i18next-http-middleware is a tool used to handle translations in web applications. A security flaw allows attackers to send specially crafted requests that can modify the application's internal configuration or cause it to crash. This could lead to service outages, corrupted website content, or the bypass of certain security checks.
Technical details
The missingKeyHandler in i18next-http-middleware (<= 3.9.6) fails to properly sanitize dotted key segments. While it blocks literal keys like '__proto__', it does not account for nested paths when a keySeparator (default '.') is used. When paired with a downstream backend like i18next-fs-backend (<= 2.6.5), these keys are passed to an unguarded setPath() function, leading to prototype pollution. An unauthenticated remote attacker can exploit this to inject properties into Object.prototype, potentially leading to RCE, DoS, or authorization bypass depending on the application environment. The issue is fixed in i18next-http-middleware 3.9.7 and i18next-fs-backend 2.6.6.
Affected products
- i18next i18next-http-middleware <= 3.9.6
Timeline
- 2026-05-22: patched: Fix released in version 3.9.7
- 2026-06-15: disclosed: NVD publication date
- 2026-06-25: advisory: GitHub Advisory published