Vue I18n: escapeParameter does not prevent XSS through srcdoc in translation messages

Summary

When escapeParameter is enabled (escapeParameterHtml in the Legacy API), Vue I18n passes the translated string through sanitizeTranslatedHtml(). The sanitizer escapes < and > in attribute values and neutralizes event handler attributes such as onerror, but it does not handle the srcdoc attribute of <iframe>.

The value of srcdoc is an HTML document: the browser decodes the character references in it and parses the result as a nested document in the same origin. The escaping and the neutralization done by the sanitizer are both character references, so they are undone inside srcdoc. This bypasses the fix for CVE-2025-53892 (GHSA-x8qp-wqqm-57ph).

Details

sanitizeTranslatedHtml() (packages/shared/src/sanitize.ts in v11.4.13) works on the final translated string:

1. It escapes <, >, " and ' inside quoted attribute values.
2. It rewrites the on of event handler attributes to on.
3. It replaces javascript: URLs in href, src, action and formaction with about:blank.

Inside srcdoc, steps 1 and 2 only produce character references, which the browser decodes before parsing the value as a document. Markup inside srcdoc therefore survives the sanitizer, with escaping or without it.

This can be reached in three ways:

• A parameter inside srcdoc. With a message such as <iframe srcdoc="{content}"></iframe>, escapeParameter escapes content into character references, and the browser decodes them again.
• Markup in a message. A message that contains an <iframe srcdoc="..."> keeps it after sanitizing. The documentation asks not to use HTML messages with content from users, but it also says that the sanitizer neutralizes event handlers in the final HTML.
• Together with GHSA-8rc2-428q-7ww3. When a call passes a list and named values, the named values are not escaped. An in a named value then reaches the sanitizer as markup, even with an ordinary message such as .