Want to protect your Mac against Zombieload (MDS) bug?
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Step one, it's not that easy to install a patch.  Apple However, installing the latest updates, which are MacOS 10.14.5 for systems running Mojave, and Security Update 2019-003 High Sierra and Security Update 2019-003 Sierra for Macs running older operating systems.
But if you think you're at increased risk of attack, Apple recommends that you disable hyper-threading on the CPU. And this comes with a tremendous performance cost, with Apple claiming "as much as a 40 percent reduction in performance with tests that include multithreaded workloads and public benchmarks."
There's a big performance here, and one that probably won't just
If you think you're at increased risk from the Zombieload attack and want to disable hyperthreading, you can do it here:
1. Apply the patches listed above.
2. Turn on or restart your Mac while holding Command (⌘) -R (or, for older Macs, one of the other MacOS Recovery key combinations) on your keyboard.
3. From Tools menu bar menu, click Terminal .
4. In the Terminal query, type the following and press RETURN:
nvram boot-args = "cwae = 2"
5. In the Terminal query, type the following and press RETURN:
nvram SMTDisable =% 01
6. Restart your Mac
To reset this change, perform an NVRAM reset (note that if you need to perform an NVRAM reset for any other reason, you must manually deactivate hypertension again).