Current Known Vulnerabilities
Existing bugs in macOS and iOS are valuable and are typically kept secret by those who might want to exploit them. 

Apple's Position on Security Issues

Apple does not disclose security issues until they are addressed. This is a deliberate decision and mentioned in security and privacy researchers:


  For the protection of our customers, Apple generally does not disclose, discuss, or confirm security issues until a full investigation is complete and any necessary patches or releases are available. Apple distributes information about security issues in its products through security advisories. You can also receive Apple security advisories through our security-announce mailing list.


Security Update Notes and CVE

Recently fixed issues are listed in Apple's security update notes and in the Common Vulnerabilities and Exposures (CVE) site.

Security Advice and Best Practices

You can find third party guides to securing macOS. Apple's own security page is a good starting point.

Guide to Securing Apple OS

See the NIST Security Configuration Checklist's Guide to Securing Apple OS X 10.10 Systems for IT Professionals for a practical list of steps and advice regarding how to secure your Macs at work.

macOS Security and Privacy Guide

Another useful guide to macOS security best practices and current threat management is drhuh's macOS-Security-and-Privacy-Guide.