Pimcore's Admin Classic Bundle provides a Backend UI for Pimcore. A potential security vulnerability has been discovered in `pimcore/admin-ui-classic-bundle` prior to version 1.3.4. The vulnerability involves a Host Header Injection in the `invitationLinkAction` function of the UserController, specifically in the way `$loginUrl` trusts user input. The host header from incoming HTTP requests is used unsafely when generating URLs. An attacker can manipulate the HTTP host header in requests to the /admin/user/invitationlink endpoint, resulting in the generation of URLs with the attacker's domain. In fact, if a host header is injected in the POST request, the $loginURL parameter is constructed with this unvalidated host header. It is then used to send an invitation email to the provided user.