Summary
i18next-http-middleware: Prototype pollution and path traversal via user-controlled language and namespace parameters
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.
Affected versions
< 3.9.3.
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.
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.
Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.
CVE-2026-41690 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.
Affected versions
Security releases
Kodem intelligence
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.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
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.
Frequently Asked Questions
- What is CVE-2026-41690? CVE-2026-41690 is a high-severity path traversal vulnerability in i18next-http-middleware (npm), affecting versions < 3.9.3. It is fixed in 3.9.3. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
- How severe is CVE-2026-41690? CVE-2026-41690 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.
- Which versions of i18next-http-middleware are affected by CVE-2026-41690? i18next-http-middleware (npm) versions < 3.9.3 is affected.
- Is there a fix for CVE-2026-41690? Yes. CVE-2026-41690 is fixed in 3.9.3. Upgrade to this version or later.
- Is CVE-2026-41690 exploitable, and should I be worried? Whether CVE-2026-41690 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
- What actually determines whether CVE-2026-41690 is exploitable, and how bad it is? 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.
- How do I fix CVE-2026-41690? Upgrade
i18next-http-middlewareto 3.9.3 or later.