The issue appears to be a glitch in the mobile OS and only occurs when a unicode text message is displayed as a banner alert. When the message is too long to display, the system attempts to abbreviate the message with an ellipsis, and if that is placed in the middle of a set of non-Latin script characters it causes the system to crash and the phone to reboot.
It’s a very specifically crafted message, and can be sent in Arabic, Chinese, or Marathi (and presumably others, these are what they’ve found so far), and is pretty easy to replicate.
There are reports that sending a specifically crafted photo can cause an erase of the source of the crash. I can’t find anything backing that up however, although there is mention one place that if you can’t use iOS’s messenger any longer you can share a photo with a friend to regain access.
Users on The Daily What are reporting that an Arabic crash string is لُلُصّبُلُلصّبُررً ॣ ॣh ॣ ॣ 冗, which as far as I can tell from Google Translate means absolutely nothing.
It seems like a fairly easy fix, but for the next few days you might want to wield sharp objects and threaten your prankster friends with them in the event your phone crashes.