If hyperthreading is not disabled, a timing attack vulnerability exists, similar to previous Spectre attacks. Apple has shipped macOS 10.14.5 with an option to disable hyperthreading in applications running untrusted code in a thread through a new sysctl. Firefox now makes use of it on the main thread and any worker threads. *Note: users need to update to macOS 10.14.5 in order to take advantage of this change.* External Reference: https://www.mozilla.org/en-US/security/advisories/mfsa2019-14/#CVE-2019-9815
Acknowledgments: Name: the Mozilla project Upstream: Multiple independent researchers