Will there be a Sierra fix or just one for High Sierra?

The Apple Security pages say that the security updates late last year included Meltdown fixes for Sierra and El Capitan as well as High Sierra. https://support.apple.com/en-au/HT208331

So not for Yosemite and older.

You must have macOS Sierra 10.12.6 installed for subsequent Sierra Security updates to appear in the Updates tab of the Mac App Store. If you have disabled these in the Mac App Store, then no Updates will reach you.

If you have some version of Sierra prior to 10.12.6, you can download, and apply the Sierra 10.12.6 Combo update which will get the base operating system to where it can receive Security updates.

There have been two Security Updates for Sierra since Apple released 10.12.6: Security Update 2017-001, and Security Update 2017-002.

Actually, No. I read the page and Meltdown and Spetre were not mentioned by name. Meltdown was referred to by its technical name, CVE-2017-5754, and appeared only in an reference to High Sierra 10.13.1. There was no mention of Spectre or its technical names: CVE-2017-5753 and CVE-2017-5715. I spent some time on the phone with Apple Support; the first two people I talked to had not heard of the issue. It was pushed up till someone in engineering knew about it. They sent me a link, https://support.apple.com/kb/HT208394, to the only official Apple report on the vulnerabilities, which described them in some detail, where I fond the names mentioned above. As for what they are doing about it, they had this to say: "https://support.apple.com/kb/HT208394We continue to develop and test further mitigations within the operating system for the Spectre techniques, and will release them in upcoming updates of iOS, macOS, and tvOS. watchOS is unaffected by Spectre."

Hello Howard,

Technically we are forbidden from speculating here in the forums about Apple's policies or future products. But I'll do it anyway 🙂.

Apple does provide regular security updates to recent OS versions. Usually those updates are limited to things like the built-in malware removal. Sometimes there are more in-depth updates. There was a security update for El Capitan less than a month ago. Yosemite had a security update last July. The last Mavericks security update was July of 2016, 3 years after it was released.

But there is no way to tell whether these current fixes, which are still in development, can be easily, safely, and/or quickly applied to previous versions of the operating system.

The page has changed since I posted. The reference to Sierra and El Capitan has been removed - I assume it was a mistake. So, for now, only High Sierra has the Meltdown fix.

Apple has said elsewhere that Spectre related fixes are coming soon. Safari update soon to mitigate potential javascript exploits.