CVE-2026-41690

18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Versions prior to 3.9.3 allow an unauthenticated HTTP client to pollute Object.prototype in the Node.js process hosting the middleware, via two unvalidated entry points that reach internal object-key writes: getResourcesHandler and missingKeyHandler. This can break authorisation checks (if (user.isAdmin) returning true for any user), cause type-confusion DoS, and depending on downstream code it can be chained into RCE.

18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Versions prior to 3.9.3 allow an unauthenticated HTTP client to pollute Object.prototype in the Node.js process hosting the middleware, via two unvalidated entry points that reach internal object-key writes: getResourcesHandler and missingKeyHandler. This can break authorisation checks (if (user.isAdmin) returning true for any user), cause type-confusion DoS, and depending on downstream code it can be chained into RCE.

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 — reads lng/ns from query parameters or route params and passes them unvalidated to:
  • utils.setPath(resources, [lng, ns], ...) — the setPath helper did not guard against __proto__, constructor, or prototype keys, writing into Object.prototype when those values were supplied.
  • i18next.services.backendConnector.load(languages, namespaces, ...) — depending on the configured backend, unvalidated path segments enabled filesystem path traversal (e.g. with i18next-fs-backend) or SSRF (e.g. with i18next-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 with for...in, which traverses inherited prototype-chain properties. A POST body like {"__proto__": {"isAdmin": true}} was forwarded into saveMissing.

Impact

• Prototype pollution — a single unauthenticated request of the form GET /locales/resources.json?lng=__proto__&ns=isAdmin writes into Object.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/ns into paths or URLs, attacker-controlled values like ns=../../etc/passwd or lng=internal-service could reach resources outside the intended scope.
• Denial of service — the unbounded i18next.options.ns growth, 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:

1. Blocks __proto__, constructor, and prototype keys in utils.setPath.
2. Replaces the for...in body iteration in missingKeyHandler with Object.keys() plus an explicit dangerous-keys guard.
3. Introduces a utils.isSafeIdentifier helper (denylist approach — still permits any legitimate i18next language code shape) that filters lng/ns values for path-traversal, path separators, control characters, prototype keys, and over-long inputs before they reach the backend connector and before they are pushed into i18next.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.

18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Versions prior to 3.9.3 allow an unauthenticated HTTP client to pollute Object.prototype in the Node.js process hosting the middleware, via two unvalidated entry points that reach internal object-key writes: getResourcesHandler and missingKeyHandler. This can break authorisation checks (if (user.isAdmin) returning true for any user), cause type-confusion DoS, and depending on downstream code it can be chained into RCE.

18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Versions prior to 3.9.3 allow an unauthenticated HTTP client to pollute Object.prototype in the Node.js process hosting the middleware, via two unvalidated entry points that reach internal object-key writes: getResourcesHandler and missingKeyHandler. This can break authorisation checks (if (user.isAdmin) returning true for any user), cause type-confusion DoS, and depending on downstream code it can be chained into RCE.

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 — reads lng/ns from query parameters or route params and passes them unvalidated to:
  • utils.setPath(resources, [lng, ns], ...) — the setPath helper did not guard against __proto__, constructor, or prototype keys, writing into Object.prototype when those values were supplied.
  • i18next.services.backendConnector.load(languages, namespaces, ...) — depending on the configured backend, unvalidated path segments enabled filesystem path traversal (e.g. with i18next-fs-backend) or SSRF (e.g. with i18next-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 with for...in, which traverses inherited prototype-chain properties. A POST body like {"__proto__": {"isAdmin": true}} was forwarded into saveMissing.

Impact

• Prototype pollution — a single unauthenticated request of the form GET /locales/resources.json?lng=__proto__&ns=isAdmin writes into Object.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/ns into paths or URLs, attacker-controlled values like ns=../../etc/passwd or lng=internal-service could reach resources outside the intended scope.
• Denial of service — the unbounded i18next.options.ns growth, 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:

1. Blocks __proto__, constructor, and prototype keys in utils.setPath.
2. Replaces the for...in body iteration in missingKeyHandler with Object.keys() plus an explicit dangerous-keys guard.
3. Introduces a utils.isSafeIdentifier helper (denylist approach — still permits any legitimate i18next language code shape) that filters lng/ns values for path-traversal, path separators, control characters, prototype keys, and over-long inputs before they reach the backend connector and before they are pushed into i18next.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.