XSS in I18next I18next-Http-Middleware

CVE-2026-41683

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.

Vulnerability class: XSS (Cross-Site Scripting)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 8.6 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L.

EPSS exploit prediction

EPSS: 0.004 (31.3th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 31st percentile — 31.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-41683: held from 0.003 to 0.004.

EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-41683?
CVE-2026-41683 is a high-severity vulnerability in I18next I18next-Http-Middleware, classified under Cross-site Scripting. CVSS score: 8.6/10. Published 2026-05-08.
How severe is CVE-2026-41683?
High severity. CVSS v3 base score is 8.6 out of 10.