Microsoft has released security updates for Microsoft Exchange servers running unsupported Cumulative Update versions vulnerable to ProxyLogon attacks. These additional security updates are meant to be installed only on machines running Exchange Server versions not supported by the original Match 2021 security patches released a week ago. Microsoft has also updated the Microsoft Safety Scanner tool to help customers detect web shells deployed in recent Exchange Server attacks by Chinese-backed state hackers. The updates are only available through the Microsoft Download Center, and you can download standalone update packages for servers running:
Source: https://www.bleepingcomputer.com/news/security/microsoft-releases-proxylogon-updates-for-unsupported-exchange-servers/