Whatsapp is facing security issues all the time. The newest
one allows MITM attacks where the attacker can alter sent
messages or hide them in a group chat from certain group
members. How the attack works shows the following video.

The company “Check Point Research” has made this video and explains how the exploit can be used and shows how the other’s chats are affected.

They call it the FakeApp vulnerability:

You may want to watch the video in fullscreen mode () in order to recognize the text.