Complex sentence: In September 2011, WhatsApp released a new version of the Messenger application for iPhones, closing critical security holes that allowed forged messages to be sent and messages from any WhatsApp user to be read.
Split into simpler sentences: In September 2011, a new version of the WhatsApp Messenger application for iPhones was released. In this new version, the developer has closed a number of critical security holes that allowed forged messages to be sent and messages from any WhatsApp user to be read.