CVE-2026-41683 is a high-severity (CVSS 8.6) Cross-site Scripting (XSS) vulnerability in i18next-http-middleware. A fix is available for i18next-http-middleware — see the affected versions and patch details below.
HTTP response splitting and DoS in i18next-http-middleware via unsanitised Content-Language header
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-41683.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
i18next-http-middlewarenpmDescription
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+maliciouscould 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
- Session fixation via an injected
- Node.js ≥ 14.6.0 — denial of service.
res.setHeader()throwsERR_INVALID_CHARwhen 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+\s*on\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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | i18next-http-middleware | all versions | 3.9.3npm install i18next-http-middleware@3.9.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for i18next-http-middleware, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update i18next-http-middleware to 3.9.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-41683 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2026-41683 in your dependencies?
Find it across npm, including transitive dependencies.