Apple released macOS High Sierra 10.13.2 Supplemental Update that includes security improvements to Safari and WebKit to mitigate the effects of Spectre vulnerability (CVE-2017-5753 and CVE-2017-5715).
Apple also provided a security update for Safari 11.0.2 for OS X El Capitan 10.11.6 and macOS Sierra 10.12.6, with fixes for the same Spectre vulnerabilities. The MacOS High Sierra update will already upgrade Safari to version 11.0.2 (13604.4.7.1.6) or version 11.0.2 (13604.4.7.10.6).
Finally, Apple issued security update for iOS 11.2.2 that includes security improvements to Safari and WebKit to also mitigate Spectre. As revealed last week, Spectre breaks the isolation between different applications running on a PC. Spectre can allow an attacker to trick error-free programs into leaking their secrets.