A particular string of character sent by text message can cause an iPhone's messaging app to crash, or shut down the phone.
The message, revealed on Reddit late Tuesday, seems to exploit an inability of the app's preview feature to correctly render the characters, which are not the standard alphanumerics or emoticons one typically texts. After repeated attempts, the app crashes, leaving the user unable to send or receive texts as usual.
If the message is received while the phone is on the lock screen, it causes the phone to reboot.
Many iPhone owners took to Twitter Wednesday complaining they were hit.
Users on Reddit reported that the text also works via the messaging app WhatsApp and that it affects Apple Watches as well as iPhones.
CNET senior editor Jeff Bakalar described the exploit as a popular prank that can only be perpetrated by someone who has your phone number. "It's probably not going to happen to you unless someone you know, who has your number, targets you," he told CBS News.
And because the string is specific and unusual, it's unlikely to be sent by accident.
There are ways to fix the lockout, or prevent it from happening.
"The person who sent you the message can send you a followup, and that seems to undo the damage," Bakalar explained. "Or, if you have a Mac, you can log into your computer and have Siri send you a text message to wake it up."
It can be avoided by disabling notification alerts, according to some Reddit posters.
Similar tricks have popped up in the past and Apple has been quick to fix them with an update.