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Anyone who controls the app’s servers could insert new people into private group chats without needing admin permission.
Once a new person is in, the phone of each member of that group chat automatically shares secret keys with that person, giving them full access to all future messages, but not past ones. It would appear as if the new member had the permission of the admin to join.Researchers Say There's a Way to Access WhatsApp Group Chats [theverge.com]