CVE-2026-41683 is a high-severity cross-site scripting (XSS) vulnerability in i18next-http-middleware (npm), affecting versions < 3.9.3. It is fixed in 3.9.3.
Summary Versions of i18next-http-middleware prior to 3.9.3 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. Impact Two concrete outcomes depending on the Node.js version: Node.js < 14.6.0, HTTP response splitting. An attacker crafting a request like GET /?lng=en%0d%0aX-Injected%3A+malicious could inject arbitrary additional HTTP response headers, enabling: Session fixation via an injected Set-Cookie Cache poisoning (injecting Location, Content-Type, etc.) Reflected XSS in controlled response bodies Node.js ≥ 14.6.0, denial of service. res.setHeader() throws ERRINVALIDCHAR when the value contains CRLF. Because the middleware did not catch this error, it propagated as an unhandled exception, returning a 500 response to all concurrent users sharing that process (in worker-pool deployments this can knock out a full server instance). The same header-setting code path fires inside the languageChanged event listener and again in the main middleware flow, so the flaw was triggered at least twice per affected request. Related (same release) Version 3.9.3 also tightens the hasXSS() regex that was designed as a secondary filter on detected language values. The previous pattern /<\s\w+\son\w+\s=.?>/i only matched event handlers in the first attribute position, so payloads like <input autofocus onfocus=alert(1)> bypassed the filter. Applications that rendered res.locals.language into HTML with a context-unsafe templating mode (EJS <%- %>, Pug !{…}, Handlebars {{{…}}}) could be XSSed despite the filter being in place. This bypass is noted here because it is fixed in the same release, but the primary vulnerability reported in this advisory is the CRLF/header-injection path above. Affected versions < 3.9.3. Patch Fixed in 3.9.3. The patch introduces utils.sanitizeHeaderValue(str) which strips \r, \n, and other C0/C1 control characters, and replaces both utils.escape(lng) call sites in lib/index.js with it. The hasXSS() regex has also been tightened to match event-handler attributes at any position. Workarounds No workaround short of upgrading. Front-proxying the middleware with a WAF rule that rejects \r/\n in query parameters, cookies, and path segments is a partial mitigation. Credits Discovered via an internal security audit of the i18next ecosystem.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2026-41683 has a CVSS score of 8.6 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.
A fixed version is available (3.9.3). Upgrading removes the vulnerable code path.
npm
i18next-http-middleware (< 3.9.3)i18next-http-middleware → 3.9.3 (npm)Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Already deployed Kodem? See CVE-2026-41683 in your environment →Upgrade i18next-http-middleware to 3.9.3 or later to resolve this vulnerability.
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CVE-2026-41683 is a high-severity cross-site scripting (XSS) vulnerability in i18next-http-middleware (npm), affecting versions < 3.9.3. It is fixed in 3.9.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
CVE-2026-41683 has a CVSS score of 8.6 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
i18next-http-middleware (npm) versions < 3.9.3 is affected.
Yes. CVE-2026-41683 is fixed in 3.9.3. Upgrade to this version or later.
Whether CVE-2026-41683 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
Upgrade i18next-http-middleware to 3.9.3 or later.