The tech giant has rushed to deal with the issue that had been identified the flaw, which lets hackers access devices through the iMessage service even if users do not click on a link or file.
The researchers revealed that the problem has affected all of Apple's operating systems and the firm have issued the security update in response to a "maliciously crafted" PDF file.
The issue was highlighted by the University of Toronto's Citizen Lab, who had previously found evidence of zero-click spyware but "this is the first one where the exploit has been captured so we can find out how it works", said researcher Bill Marczak.
The team revealed that the previously unknown problem has affected all major Apple devices, including iPhones, Macs and Apple Watches.
Security experts have said that although the discovery is significant most users shouldn't be too concerned as such threats are usually highly targeted.
Apple explained in a blog post that it had issued the iOS 14.8 and iPadOS 14.8 software patches after it became aware that the flaw "may have been actively exploited".
The update comes as the company prepare to launch new devices at its annual event on Tuesday (14.09.21), where new iPhones and updates to its AirPods and Apple Watch.