Whats Up with WhatsApp?

WhatsApp allows anyone who knows your phone number to look up your public details on the app, assuming you have an account. So what's to prevent someone from just iterating through all the 63 billion of so potential phone numbers in the world and finding all the people with WhatsApp accounts? Nothing.

That's the problem with systems on this scale. The researchers were probing the system with 100 million API requests per hour, for weeks, from a single IP address, and nobody noticed.