Do you know what a “Class 0” text message is? A Class 0 or “flash message” pops up immediately as a message window that the user is supposed to decide whether or not to save on the Nexus 4 or Nexus 5 phone.

An attacker can pile message upon message on a victim until the phone begins to misbehave. The Nexus 4 and Nexus 5 don't give audio notifications of incoming flash messages.

The phone will stop responding to the network if the victim's phone has SIM PIN-protection enabled.

Google has known of the flax for more than a year. As a work around you can download an app that will act as a virtual fire-wall to Class 0 messages.