The popular mobile messaging app WhatsApp is vulnerable to hijacking exposing hundreds of Millions of users vulnerable to attack. Attack relies on the way the account setup mechanism works are implemented by WhatsApp. Attack also works in case the mobile device is locked, it doesnt exploit any vulnerability in the popular messaging app. Attackers can easily take over a WhatsApp account when the attacker has the phone number of the victim., even if it is locked. It is a good practice not to leave it unattended for longer durations.”]
Source: https://securityaffairs.co/wordpress/37569/hacking/easily-hack-whatsapp-account.html