Hijack Whatsapp with your iPhone

Hijack (someone else’s) Whatsapp with your iPhone
If you want to hijack someone else’s Whatsapp and receive messages addressed to that person with your iPhone, read on. (You don't have an iPhone? see bottom)

When you install Whatsapp on your iPhone, the Whatsapp application makes contact with the Whatsapp servers, and the Whatsapp servers will send you a verification sms with a code in it. Straight from that point a counter will start counting in the Whatsapp application. Within this time Whatsapp expects you to receive your verification SMS. If this period expires Whatsapp offers you several other authentication methods. (see below)

Here you choose for the option “SMS”. And you will have to fill in your email adress:

Your Phone will now start sending an SMS to the whatsapp servers for verification. You can cancel this, as it is not necessary.
What you’re going to do next is called SMS-spoofing. You can do this via many sites on the web. Choose one, and make up your fake SMS as shown in the picture below:

To: +447900347295
From: +(Country code)(mobile number)
Message: (your email adress)
That’s all! Within minutes you will receive the activation code in your email to activate whatsapp on your iPhone with someone else’s Telephone number, and from that moment on you will receive message’s addressed to that person on your iPhone.

The only way for Whatsapp to solve this issue is sending the verification SMS from their own servers and no other way.

If you have anything other than an iPhone your also able to Hijack someone else's Whatsapp. It's even easier for you.
All other systems will start sending an SMS verification immediately from your own mobile phone! So you disconnect your mobile phone, try to send the verification sms, which is impossible since you disconnected it. Check your outbox. There you will see the verification sms. Copy that whole sms to a website where you can spoof SMS. State the FROM field as the person's Whatsapp you want to hijack, and fill in your own mobile number in the TO field.
Thats it.