http://afritechnet..ae/2015/06/how-to-hack-whatsapp-account.html
How to hack a Whatsapp Account


Whatsapp was purchased by Facebook for over $20 billion last year as a result of its popularity. It is this user popularity that has made it a prime target for hackers looking to make their mark.

It is now understood that a Whatsapp account can be hijacked without unlocking or knowing a device password - something that should be of grave concern to the hundreds of millions Whatsapp users.

The following trick will allow anyone in the vicinity of a Whatsapp user to potentially gain control over the victim's Whatsapp account with just the phone number and access to the phone for just a few seconds, even if it's locked:

Download Whatsapp and create a new account
When creating the new account use your victim’s number
In the process of creating the account ask WhatsApp to call the victim number to provide you with the PIN code.
Accept the Whatsapp call from the victim's phone

Although it affects all Whatsapp users irregardless of OS, the hack is made easier if the victim is an iPhone user with Siri authentication from the lock screen. In such a scenario the victim's contact details will be made available to access Siri's settings, meaning that an attacker can access them sans PIN code. This simplifies the process of finding out the victim's phone number.

http://afritechnet..com/2015/06/how-to-hack-whatsapp-account.html

This is more of a social engineering than a hack. The weak authentication via availability of a mobile device within a specific window makes it even easier to implement.