Potential XSS vulnerability in Action View

There is a potential Cross-Site Scripting (XSS) vulnerability in Action
View’s translation helpers. Views that allow the user to control the
default (not found) value of the t and translate helpers could be
susceptible to XSS attacks.

Impact

When an HTML-unsafe string is passed as the default for a missing
translation key named html or ending in _html,
the default string is incorrectly marked as HTML-safe and not escaped.
Vulnerable code may look like the following examples:

<%# The welcome_html translation is not defined for the current locale: %>
<%= t("welcome_html", default: untrusted_user_controlled_string) %>

<%# Neither the title.html translation nor the missing.html translation is defined for the current locale: %>
<%= t("title.html", default: [:"missing.html", untrusted_user_controlled_string]) %>

Workarounds

Impacted users who can’t upgrade to a patched Rails version can avoid
this issue by manually escaping default translations with the
html_escape helper (aliased as h):

<%= t("welcome_html", default: h(untrusted_user_controlled_string)) %>