To minimize or avoid impacts of this situation, Microsoft highly recommends that clients take immediate action to apply the patches for any on-premises Exchange deployments they have or are being managed by partners or to receive the steps they need to take to mitigate. The priority being servers which are accessible from the Internet (e.g., servers publishing Outlook on the web/OWA and ECP). 

To patch these vulnerabilities, clients should move to the latest Exchange Cumulative Updates and then install the relevant security updates on each Exchange Server. 

• You can use the Exchange Server Health Checker script, which can be downloaded from GitHub (use the latest release).
• Running this script will tell you if you are behind on your on-premises Exchange Server updates (note that the script does not support Exchange Server 2010). 
• We also recommend that your security team assess whether or not the vulnerabilities were being exploited by using the Indicators of Compromise we shared here.

Exchange patch information

• March 2, 2021 Security Update Release – Release Notes – Security Update Guide – Microsoft
• CVE-2021-26855| Microsoft Exchange Server Remote Code Execution Vulnerability (public) 
• CVE-2021-26857| Microsoft Exchange Server Remote Code Execution Vulnerability (public) 
• CVE-2021-26858| Microsoft Exchange Server Remote Code Execution Vulnerability (public) 
• CVE-2021-27065| Microsoft Exchange Server Remote Code Execution Vulnerability (public) 

We at Synergy Advisors are committed to working with clients through this issue so please let us know if you need help ASAP.