CVE-2026-115730 is a low-severity vulnerability in i18next-http-backend versions 4.0.2 up to but not including 4.0.3. It involves an incomplete fix in the interpolateUrl() function that allows attackers controlling the namespace (ns) value to redirect translation requests by supplying a namespace starting with a slash. This can lead to loading attacker-hosted translations, enabling UI content injection and cross-site scripting (XSS).
CVE-2026-105800 is a Server-Side Request Forgery (SSRF) vulnerability in the i18next-http-backend library. It occurs when an attacker controls the language or namespace input that is interpolated into the loadPath template, allowing injection of a colon character that causes the backend to request an unintended origin. This vulnerability affects versions of i18next-http-backend prior to 4.0.2 and was caused by an incomplete patch for a previous related vulnerability. The issue is fixed by rejecting colon characters in language and namespace values and ensuring URLs remain within the intended origin.
A prototype pollution vulnerability exists in i18next-http-middleware versions prior to 3.9.7. The middleware's missingKeyHandler blocks literal keys like __proto__, constructor, and prototype but does not block dotted variants such as "__proto__.polluted". When used with downstream backends like i18next-fs-backend ≤ 2.6.5 that split keys on a configured separator and write to Object.prototype without proper guards, this can lead to remote prototype pollution. This may cause crashes, corrupted translation behavior, configuration poisoning, or bypasses of property-based security checks. The issue is fixed in version 3.9.7.
Versions prior to 2.6.6 are vulnerable to prototype pollution via crafted missing-key strings when used to persist missing translation keys (e.g. via i18next-http-middleware's missingKeyHandler exposed to untrusted input). Backend.writeFile() splits each queued missing-key string on the configured keySeparator (default .) before calling the internal setPath() walker. The walker (getLastOfPath in lib/utils.js) did not guard against unsafe segments, so a key like "__proto__.polluted" was split into ["__proto__", "polluted"] and walked straight into Object.prototype, allowing an attacker to write arbitrary properties onto the global object prototype. Depending on the host application, polluted prototype properties may cause crashes, corrupted translation behaviour, configuration poisoning, or bypasses of property-based security checks. Applications are affected only if the missingKeyHandler (or another route that forwards untrusted request bodies to i18next.t(..., { ... }) with saveMissing: true) is reachable by untrusted users and the default behaviour of splitting missing-key strings on keySeparator is in use (i.e. keySeparator is not false). Apps that do not expose missing-key persistence to untrusted input are not directly affected through this attack path. This issue has been fixed in version 2.6.6. If developers using the library are unable to upgrade immediately, they should take the following precautions: do not expose i18next-http-middleware's missingKeyHandler to untrusted users (mount it behind authentication, or remove the route), disable missing-key persistence (saveMissing: false, or no backend.create implementation) when accepting writes from untrusted input, and set keySeparator: false in their i18next options to disable backend key splitting (note: this also disables nested translation keys).
i18next-fs-backend is a backend layer for i18next using in Node.js and for Deno to load translations from the filesystem. Prior to version 2.6.4, i18next-fs-backend substitutes the lng and ns options directly into the configured loadPath / addPath templates and then read / write the resulting file from disk. The interpolation is unencoded and unvalidated, so a crafted lng or ns value — containing .., a path separator, a control character, a prototype key, or simply an unexpectedly long string — allows an attacker who can influence either value to read or overwrite files outside the intended locale directory. When lng / ns are derived from untrusted input (request-scoped i18next instances behind an HTTP layer such as i18next-http-middleware, or any framework that lets the end user pick the language via query string, cookie, or header), a single request such as ?lng=../../../../etc/passwd causes the backend to attempt to read that path. This issue has been patched in version 2.6.4.
i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Prior to version 3.9.3, i18next-http-middleware passes the user-controlled lng and ns values from getResourcesHandler directly into i18next.services.backendConnector.load(languages, namespaces, …) without any sanitization. Depending on which backend is configured, the unvalidated path segments enable either path traversal or SSRF. This issue has been patched in version 3.9.3.
i18next-http-middleware is a middleware to be used with Node.js web frameworks like express or Fastify and also for Deno. Prior to version 3.9.3, i18next-http-middleware wrote user-controlled language values into the Content-Language response header after passing them through utils.escape(), which is an HTML-entity encoder that does not strip carriage return, line feed, or other control characters. When the application used an older i18next (< 19.5.0) that still exercised the backward-compatibility fallback at LanguageDetector.js:100 or otherwise produced a raw detected value, CRLF sequences in the attacker-controlled lng parameter reached res.setHeader('Content-Language', ...) verbatim. This issue has been patched in version 3.9.3.
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.
CVE-2026-41691 is a path traversal and URL injection vulnerability in i18next-http-backend versions prior to 3.0.5. The issue arises because the library interpolates language (lng) and namespace (ns) values directly into URL templates without encoding or sanitization. This allows an attacker controlling these inputs to manipulate the request URL structure, potentially accessing restricted paths. The vulnerability has been fixed in version 3.0.5. Until upgrading, users can mitigate risk by sanitizing lng and ns inputs to remove dangerous characters and limit length.
i18nextify is a JavaScript library that adds website internationalization via a script tag, without source code changes. Versions prior to 4.0.8 substitute {{key}} interpolation tokens inside src and href attribute values with the raw string returned by i18next.t(). The substitution logic in src/localize.js (the replaceInside handler) only guards against a duplicated http:// origin prefix — it does not validate the URL scheme of the substituted value. A translated value such as javascript:alert(1) or data:text/html,<script>...</script> is applied unchanged to the live DOM attribute when an attacker can influence the content of a translation file or the translation-backend response — for example, via a compromised translation CDN, user-contributed locales, a MITM on a plain-HTTP backend, or write access to the translation JSON. This issue was patched in version 4.0.8.