Apple is reacting as soon as they can when their system is in danger.
The problem comes from Webkit which allows malicious websites to execute a code on devices such as iPhone 6 and later, iPad Pro (all models), iPad Air 2 and later, iPad 5th generation and later, iPad mini 4 and later, and iPod touch (7th generation).
The company knows about the problem, but didn't say how many devices have been affected.
Thanks to the newest updates the problem has been fixed and you can sleep calmy if you own any of the above mentioned devices.
The new system versions for both iPhones and iPads has a numer 15.3.1.