According to Silvanovich, “On a Mac, this causes soagent to crash and respawn, but on an iPhone, this code is in Springboard. Receiving this message will case Springboard to crash and respawn repeatedly, causing the UI not to be displayed and the phone to stop responding to input. This condition survives a hard reset, and causes the phone to be unusable as soon as it is unlocked. The only way I could find to fix the phone is to reboot into recovery mode and do a restore. This causes the data on the device to be lost though.”
However, the good news is that this issue was patched in iOS 12.3 which means that as long as you have updated your iPhone to iOS 12.3 or later, then you should be covered. For those who have yet to update their iOS devices, then now might be a good time as any to make sure that your iPhone is running the latest update.
As we said, this is not the first time that an iMessage bug has been discovered. For example, back in 2015, there was a bug that would cause the phone to crash continuously, and there was another bug discovered in 2018 that when users sent a weird character via iMessage, it would cause the app to crash.