Apple warned of serious security vulnerabilities for iPhones, iPads and Macs that could potentially allow attackers to take complete control of these devices.
The tech giant said it is "aware of a report that this issue may have been actively exploited".
It said the malicious application "may be able to execute arbitrary code with kernel privileges", which means the hacker could fully access the device.
The vulnerability could also process "maliciously crafted web content" which "may lead to arbitrary code execution" on WebKit. WebKit is Apple's browser engine used on various applications such as Safari, Mail and App Store.
Security experts have advised users to update affected devices, such as the iPhones6S and later models, several models of the iPad, including the fifth generation and later, all iPad Pro models and the iPad Air 2, as well as Mac computers running MacOS Monterey. It also affects some iPod models.
Rachel Tobac, chief of SocialProof Security, said that the vulnerability means a hacker could get "full admin access to the device" so that they can "execute any code as if they are you, the user".
Those who should be particularly attentive to updating their software are "people who are in the public eye" such as activists or journalists who might be the targets of sophisticated nation-state spying, Ms Tobac said.
Apple released two security reports about the issue on Wednesday, although they did not receive wide attention outside of tech publications.