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.. This vulnerability affects Thunderbird < 60.7, Firefox < 67, and Firefox ESR < 60.7.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-9815
• https://bugzilla.mozilla.org/show_bug.cgi?id=1546544
• https://mdsattacks.com/
• https://www.mozilla.org/security/advisories/mfsa2019-13/
• https://www.mozilla.org/security/advisories/mfsa2019-14/
• https://www.mozilla.org/security/advisories/mfsa2019-15/