FreeScout is a free self-hosted help desk and shared mailbox. Prior to version 1.8.179, when creating a conversation from a message in another conversation, there is no check to ensure that the user has the ability to view this message. Thus, the user can view arbitrary messages from other mailboxes or from other conversations to which they do not have access (access restriction to conversations is implemented by the show_only_assigned_conversations setting, which is also not checked). This issue has been patched in version 1.8.179.