“Microsoft has detected multiple 0-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks. In the attacks observed, the threat actor used these vulnerabilities to access on-premises Exchange servers which enabled access to email accounts, and allowed installation of additional malware to facilitate long-term access to victim environments.”
As these vulnerabilities are currently being exploited and used in targeted attacks, patching should be done as soon as possible.
Along with attack details and information about these vulnerabilities, Microsoft also published how to scan exchange log files for indicators of compromise, which is also recommended to do.
Update 2020-03-07: There are currently many published exploits for this vulnerability. Patching this vulnerability is not enough, one must also investigate for potential breaches.
Internally this is being tracked in BF-VLN-2229454.