i18nextify has DOM XSS via javascript:/data: URL schemes in translated href/src attributes

Summary

Versions of i18nextify 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 (replaceInside handler around line 122) 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.

Impact

When an attacker can influence the content of a translation file or the translation-backend response — compromised translation CDN, user-contributed locales, MITM on a plain-HTTP backend, write access to the translation JSON — they can:

• Set any href on an anchor to a javascript: URI, executing arbitrary JavaScript when the victim clicks the link.
• Set any src on <iframe>, <object>, or <embed> to a data:text/html URI containing a full script payload that runs in the page's origin.
• Use vbscript: on legacy IE installations or file: for local-resource navigation attacks.

This path is distinct from the general i18nextify design that intentionally renders HTML from translations — href/src schemes are narrow and attack-specific, and no legitimate translation needs javascript: or data:. The fix therefore blocks these schemes outright without changing other behaviour.

Also fixed in 4.0.8

• debug / saveMissing URL-parameter substring match. The previous detection window.location.search.indexOf('debug=true') > -1 matched the substring anywhere in the query string. A URL like ?nosaveMissing=true silently enabled saveMissing mode, causing the victim's browser to POST every unknown translation key to the configured addPath — a form of CSRF-style abuse of missing-key reporting. ?track_debug=true enabled verbose debug logging, leaking i18next internals to the console. Now uses URLSearchParams for exact parameter matching.
• Optional sanitize(html, ctx) hook. The library's core purpose is to render HTML from translations — a behaviour that is safe only when the translation source is fully trusted. Applications with partially-trusted sources (user-contributed locales, third-party CDN, MITM-exposed HTTP backend) can now wire a sanitizer (e.g. DOMPurify) via i18next.options.sanitize. Defaults to pass-through to preserve existing behaviour for the main use case.

Affected versions

All versions of i18nextify prior to 4.0.8.

Patch

Fixed in 4.0.8. The URL-scheme blocklist is ^\s*(javascript|data|vbscript|file)\s*: (case-insensitive) applied to each translated value before it is joined back into the href/src attribute. Values matching the blocklist are replaced with an empty string so the attribute becomes harmless rather than leaving the attacker's URL in place.

Workarounds

No workaround short of upgrading. If you cannot upgrade immediately, audit every translation file for javascript:, data:, vbscript:, and file: prefixes in any value that may reach an href/src position, and restrict translation-file write access to trusted operators. Serving translations over HTTPS and pinning the translation backend to an internal origin reduce the MITM surface.

Credits

Discovered via an internal security audit of the i18next ecosystem.