CVE-2026-41683: i18next i18next-HTTP-Middleware

High severity, CVSS 8.6. EPSS: 0.4% chance of exploitation in the next 30 days.

i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Prior to version 3.9.3, i18next-http-middleware wrote user-controlled language values into the Content-Language response header after passing them through utils.escape(), which is an HTML-entity encoder that does not strip carriage return, line feed, or other control characters. When the application used an older i18next (< 19.5.0) that still exercised the backward-compatibility fallback at LanguageDetector.js:100 or otherwise produced a raw detected value, CRLF sequences in the attacker-controlled lng parameter reached res.setHeader('Content-Language', ...) verbatim. This issue has been patched in version 3.9.3.

Affected products

• i18next i18next-HTTP-Middleware: before 3.9.3 (fixed in 3.9.3)

Published 2026-05-08. Last modified 2026-06-17.