WhatsApp has been found to be prone to hijacking without unlocking or knowing your device password, making its hundreds of Millions of users vulnerable to, not just hackers, but also non-technical people. The attacker needs nothing more than a phone number of the target person and access to the target mobile phone for a few seconds, even if it is locked. The trick enables the offender to get full control over the victim’s WhatsApp account in no time. It independently works on all mobile platforms, including Android, Windows and Apple’s iOS.
Source: https://thehackernews.com/2015/06/hijacking-whatsapp-account.html