News of the bug emerged after reports in Italian blog Mobile World and has since been widely replicated, with numerous confirmations posted online.
The character is from the Telugu language, a Dravidian language spoken in India by about 70 million people, roughly five per cent of the country's population.
As apps try and fail to load the character, users can get trapped in an endless loop of crashes.
The only way to escape this cycle is to get someone else to message you and delete the thread containing the malicious message.
Writing on Mobile World, tech blogger Giuseppe Tripodi said: "The error we are talking about presents an Indian character that, if received or simply pasted in a text field, can lead to freeze of applications or crashes of the entire operating system.
"If the character is displayed within an application, the app in question will crash and will continue to close each time you try to start it.
"The situation gets worse if someone sends you the symbol and iOS tries to show it in a notification.
"In this case, the entire Springboard will be blocked.
"If this happens, wait and hope that the Springboard will restart properly, because if you force a reboot of the device, it will bootloop."
Telegram and Skype do not appear to be affected and the issue couldn't be recreated in the public beta versions of iOS 11.3.
That suggests Apple is aware of the problem and will fix it once iOS 11.3 is released publicly.
This is expected to happen sometime in the next few months.
