HTTP response splitting and DoS in i18next-http-middleware via unsanitised Content-Language header

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

8 May 2026 15:27:05 UTC

Source modified

8 May 2026 23:29:00 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 8.6 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

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.

Source published

May 8, 2026, 3:27:05 PM (timezone not stated)

Source modified

May 8, 2026, 11:29:00 PM (timezone not stated)

Observed here

7 Nov 2026 09:31:27 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 8.6 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

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.

Source published

8 May 2026 16:16:11 UTC

Source modified

26 Oct 2026 18:10:00 UTC

Observed here

27 Oct 2026 10:10:19 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.6 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

### 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 `ERR_INVALID_CHAR` 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+\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.

Source published

22 Apr 2026 20:25:49 UTC

Source modified

13 May 2026 13:52:05 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Publisher rating · HIGH