Whatsapp users have been able to invite other users to a group chat using a link. If you open a link invitation, you will automatically be added to the group. As Vice reports, until recently those links were anything but private – if you search Google for the right URL, thousands of invitation links can be found.
Jordan Wildon made a broader public aware of this: “Whatsapp group chats are not as secure as users think,” he wrote on Twitter. He noticed that Google indexes those WhatsApp invitation URLs and lists them when searching. This makes it possible to join any group, because clicking the link is enough to join a group. After becoming a member of the group, you will automatically see all the phone numbers of the other participants.