Joomla Extension - joomshaper.com - Open Redirect via Base64 Return Parameter in Helix Ultimate < 2.2.10 - Return redirect parameters accepted arbitrary Base64 strings without verifying whether the resolved target was an internal site URL via Uri::isInternal.
URL Redirection to Untrusted Site The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.