Microsoft has released a new, one-click mitigation tool, Microsoft Exchange On-Premises Mitigation Tool to help customers who do not have dedicated security or IT teams to apply these security updates. We have tested this tool across Exchange Server 2013, 2016, and 2019 deployments. This new tool is designed as an interim mitigation for customers who are unfamiliar with the patch/update process or who have not yet applied the on-premises Exchange security update.
By downloading and running this tool, which includes the latest Microsoft Safety Scanner, customers will automatically mitigate CVE-2021-26855 on any Exchange server on which it is deployed. This tool is not a replacement for the Exchange security update but is the fastest and easiest way to mitigate the highest risks to internet-connected, on-premises Exchange Servers prior to patching.