CVE-2026-41690 — i18next-http-middleware
HIGHCVE-2026-41690 is a high-severity (CVSS 8.6) Path Traversal vulnerability in i18next-http-middleware. A fix is available for i18next-http-middleware — see the affected versions and patch details below.
Prototype pollution and path traversal in i18next-http-middleware via user-controlled language and namespace parameters
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-2026-41690.
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 pass user-controlled lng and ns parameters to two internal paths that use them in ways that enable prototype pollution and, depending on the configured backend, path traversal or SSRF.
The vulnerable entry points are unauthenticated HTTP handlers that are part of the middleware's public API:
getResourcesHandler— readslng/nsfrom query parameters or route params and passes them unvalidated to:utils.setPath(resources, [lng, ns], ...)— thesetPathhelper did not guard against__proto__,constructor, orprototypekeys, writing intoObject.prototypewhen those values were supplied.i18next.services.backendConnector.load(languages, namespaces, ...)— depending on the configured backend, unvalidated path segments enabled filesystem path traversal (e.g. withi18next-fs-backend) or SSRF (e.g. withi18next-http-backend).- A
namespaces.forEach(ns => i18next.options.ns.push(ns))loop additionally performed permanent, unbounded growth of the shared singleton namespace list.
missingKeyHandler— iterated the incoming request body withfor...in, which traverses inherited prototype-chain properties. A POST body like{"__proto__": {"isAdmin": true}}was forwarded intosaveMissing.
Impact
- Prototype pollution — a single unauthenticated request of the form
GET /locales/resources.json?lng=__proto__&ns=isAdminwrites intoObject.prototype, affecting every plain object created subsequently in the Node.js process. This can break authorization checks (if (user.isAdmin)), cause denial of service via type confusion, or be chained into RCE depending on what downstream code reads from polluted objects. - Path traversal / SSRF — with filesystem or HTTP backends that interpolate
lng/nsinto paths or URLs, attacker-controlled values likens=../../etc/passwdorlng=internal-servicecould reach resources outside the intended scope. - Denial of service — the unbounded
i18next.options.nsgrowth, plus repeated backend load calls, enabled memory and CPU exhaustion from unique namespace payloads.
Affected versions
< 3.9.3.
Patch
Fixed in 3.9.3. The patch:
- Blocks
__proto__,constructor, andprototypekeys inutils.setPath. - Replaces the
for...inbody iteration inmissingKeyHandlerwithObject.keys()plus an explicit dangerous-keys guard. - Introduces a
utils.isSafeIdentifierhelper (denylist approach — still permits any legitimate i18next language code shape) that filterslng/nsvalues for path-traversal, path separators, control characters, prototype keys, and over-long inputs before they reach the backend connector and before they are pushed intoi18next.options.ns.
Workarounds
No workaround short of upgrading. Front-proxying the middleware with a WAF rule that rejects requests containing __proto__, constructor, prototype, .., or control characters in lng/ns query parameters or body keys 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-2026-41690 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.
Frequently Asked Questions
Is CVE-2026-41690 in your dependencies?
Find it across npm, including transitive dependencies.