Today, Microsoft has taken additional steps to further support their customers who are still vulnerable and have not yet implemented the complete security update. With the latest security intelligence update, Microsoft Defender Antivirus and System Center Endpoint Protection will automatically mitigate CVE-2021-26855 on any vulnerable Exchange Server on which it is deployed. Customers do not need to take action beyond ensuring they have installed the latest security intelligence update (build 1.333.747.0 or newer), if they do not already have automatic updates turned on.
The Exchange security update is still the most comprehensive way to protect your servers from these attacks and others fixed in earlier releases. This interim mitigation is designed to help protect customers while they take the time to implement the latest Exchange Cumulative Update for their version of Exchange.
More details here: https://www.microsoft.com/security/blog/2021/03/18/automatic-on-premises-exchange-server-mitigation-now-in-microsoft-defender-antivirus/
