As you probably already heard in recent news, 1,000,001 Apple UDID’s were leaked. It’s unfortunate that so many apps use UDID’s to identify users since it’s extremely insecure.
This brings me to WhatsApp, a free messaging service, used by millions of people. Their system runs on a modified version of XMPP (Extensible Messaging and Presence Protocol). There is nothing wrong with using XMPP, but there is a problem in how WhatsApp handle authentication.