First things first, chances are good that your Mac is not at risk in the first place. Want to check?

• Go to the Apple menu, select About This Mac
• Click the System Report… button
• Click the Hardware title in the sidebar

In the Hardware Overview section (it’s relatively short), look for the term hyper-threading. Don’t see it? You can ignore this issue.

If your Mac does support hyper-threading, you should take a look at this official Apple Support document.

Before you take any action, note:

Testing conducted by Apple in May 2019 showed as much as a 40 percent reduction in performance with tests that include multithreaded workloads and public benchmarks. Performance tests are conducted using specific Mac computers. Actual results will vary based on model, configuration, usage, and other factors.

And:

Although there are no known exploits affecting customers at the time of this writing, customers who believe their computer is at heightened risk of attack can use the Terminal app to enable an additional CPU instruction and disable hyper-threading processing technology, which provides full protection from these security issues.

No known exploits. Just the potential for exploits. Forewarned is forearmed.

A bit of a PR ding for Apple, but a mighty issue for Intel.

UPDATE: There is some question as to whether lack of hyper-threading support puts your Mac in safe harbor from the Intel vulnerability. The issue is, if you disable hyper-threading (and add that extra CPU instruction), you’ll be safe. But the question is, if your Mac doesn’t support hyper-threading in the first place, does that mean your Mac is not susceptible to the issue?

If anyone knows the definitive answer to this, please reach out.