vue-i18n's escapeParameterHtml does not prevent DOM-based XSS through its tag attributes

Summary

The escapeParameterHtml: true option in Vue I18n is designed to protect against HTML/script injection by escaping interpolated parameters. However, this setting fails to prevent execution of certain tag-based payloads, such as <img src=x onerror=...>, if the interpolated value is inserted inside an HTML context using v-html.

This may lead to a DOM-based XSS vulnerability, even when using escapeParameterHtml: true, if a translation string includes minor HTML and is rendered via v-html.

Details

When escapeParameterHtml: true is enabled, it correctly escapes common injection points.

However, it does not sanitize entire attribute contexts, which can be used as XSS vectors via:

<img src=x onerror=alert(1)>

PoC

In your Vue I18n configuration:

const i18n = createI18n({
  escapeParameterHtml: true,
  messages: {
    en: {
      vulnerable: 'Caution: <img src=x onerror="{payload}">'
    }
  }
});

Use this interpolated payload:

const payload = '<script>alert("xss")</script>';
Render the translation using v-html (even not using v-html):

<p v-html="$t('vulnerable', { payload })"></p>
Expected: escaped content should render as text, not execute.

Actual: script executes in some environments (or the payload is partially parsed as HTML).

Impact

This creates a DOM-based Cross-Site Scripting (XSS) vulnerability despite enabling a security option (escapeParameterHtml) .