Apple is urging iPhone and iPad users to update their devices to fix security flaws that might have been "actively exploited" by hackers.
Apple made the software upgrades available on Tuesday, adding a rare note suggesting it was a serious threat.
The company credited anonymous researchers for pointing out the vulnerability but provided little details about the nature of the threat.
The company said one of the vulnerabilities affects the "kernel," the nerve centre of Apple's operating system iOS.
Two others affect WebKit, the web browser engine used by Safari and other apps.
The upgrade is for several generations of devices, starting with the iPhone 6.
Meanwhile, Apple topped Amazon and Google to become the world's most valuable brand for the first time since 2016 after its brand value jumped 87 per cent to $US263.4 billion ($A343.3 billion), according to the Brand Finance Global 500 2021 ranking by London-based Brand Finance, a brand valuation consultancy.
Australian Associated Press