An attacker can secretly eavesdrop on your private end-to-end encrypted group chats on WhatsApp, Threema and Signal messaging apps. German researchers discovered that both Signal and WhatsApp fail to properly authenticate an entity that is adding a new member to the group, this means that an unauthorized user that is not a group administrator or even a member of the group can add a member. WhatsApp fixed one problem with a feature of their encryption that made it harder to crack future messages even after an attacker obtained one decryption key.”]
Source: https://securityaffairs.co/wordpress/67627/hacking/whatapp-group-chat-hack.html