Spectre breaks the isolation between different applications. It allows an attacker to trick error-free programs, which follow best practices, into leaking their secrets. In fact, the safety checks of said best practices actually increase the attack surface and may make applications more susceptible to Spectre
Spectre is harder to exploit than Meltdown, but it is also harder to mitigate.
Microsoft has issued a patch for Windows 10, while other versions of Windows are expected to be patched on the traditional Patch Tuesday on January 9, 2018. Microsoft has also issued a guidance document for mitigations on client devices. Please note that the patches released by Microsoft may be incompatible with certain antivirus software.
MacOS 10.13.2 mitigates some of the disclosed vulnerabilities, but MacOS 10.13.3 will enhance or complete these mitigations.