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

published 2026-04-22 6 mo ago modified 2026-10-26 13 d ago also EUVD-2026-28791 , GHSA-c3h8-g69v-pjrg

cvss: 8.6; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 26%
public exploits: 0; none known

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.

Patch / fix

fixed in 2 links 1

npm i18next-http-middleware 3.9.3
i18next i18next-http-middleware 3.9.3

github.com/i18next/i18next-http-middleware@65301c1 commit

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 6 mo ago (2026-05-08)

Exploit prediction

0.33% chance of exploitation in the next 30 days, higher than 26% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.6	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L
GitHub	3.1	8.6	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Affected packages

ecosystem	package	affected	fixed in
npm	i18next-http-middleware	<3.9.3	3.9.3

Affected products

i18next i18next-http-middleware

Classification

Weakness

CWE-79 Improper Neutralization of Input During Web Page Generation
CWE-113 Improper Neutralization of CRLF Sequences in HTTP Headers

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary