https://arstechnica.com/information-technology/2020/08/update-tuesday-fixes-2-0days-and-botched-patch-for-a-backdoor-threat/

Microsoft on Tuesday patched 120 vulnerabilities, two that are notable because they’re under active attack and a third because it fixes a previous patch for a security flaw that allowed attackers to gain a backdoor that persisted even after a machine was updated.

Zero-day vulnerabilities get their name because an affected developer has zero days to release a patch before the security flaw is under attack. Zero-day exploits can be among the most effective because they usually go undetected by antivirus, intrusion prevention systems, and other security protections. These types of attacks usually indicate a threat actor of above-average means because of the work and skill required to identify the unknown vulnerability and develop a reliable exploit. Adding to the difficulty: the exploits must bypass defenses developers have spent considerable resources implementing.