Apple has released an update to macOS High Sierra for all Macs running macOS 10.13.2. The supplemental security update likely addresses the Spectre flaw that affected Safari and may contain further mitigations for Meltdown.
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Apple released the original version of macOS 10.13.2 back in early December, but acknowledged last week that Safari would require further updates to help lessen potential issues related to recently disclosed vulnerabilities.
All Mac systems and iOS devices are affected, but there are no known exploits impacting customers at this time. Since exploiting many of these issues requires a malicious app to be loaded on your Mac or iOS device, we recommend downloading software only from trusted sources such as the App Store.
Apple has already released mitigations in iOS 11.2, macOS 10.13.2, and tvOS 11.2 to help defend against Meltdown. Apple Watch is not affected by Meltdown. In the coming days we plan to release mitigations in Safari to help defend against Spectre. We continue to develop and test further mitigations for these issues and will release them in upcoming updates of iOS, macOS, tvOS, and watchOS.