This indicates an attack attempt to exploit a Denial of Service vulnerability in Apple iOS.
The vulnerability is due to a lack of input validation in the application when handling a crafted text message. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
iPhone 5s and later, iPad Air and later, and iPod touch 6th generation using iOS 11.2.4 or earlier
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.