Due to the severity of the HAFNIUM exploit, Microsoft has released further updates for older Exchange Server versions. However, the updates cannot be obtained via Windows Update, but must be downloaded and installed manually.
Further information on the updates can be found here:
March 2021 Exchange Server Security Updates for older Cumulative Updates of Exchange Server
Some have assumed that older Exchange versions are not vulnerable to HAFNIUM exploits, but this is not correct. However, Microsoft only ever supports two versions of Exchange CUs: Specifically, this means that only the current and the previous CU are supported, so for Exchange 2016, for example, there are only updates for CU 18 and CU 19. However, since older versions are also vulnerable, Microsoft has now also released updates for older Exchange versions:
However, no updates are yet available for other Exchange versions. The best way would therefore be to install the latest CU as quickly as possible and the HAFNIUM exploit security updates immediately afterwards. Now that a few days have passed, the servers should be carefully checked for an attack.
The newly provided updates can be found here:
Under the article to the original article you will find a lot of valuable information from admins who were / are affected and have already initiated countermeasures:
https://www.frankysweb.de/exchange-server-neue-sicherheitsupdates-maerz-2021/
If you are also affected, then take a look at the article on reinstalling only, sometimes a reinstallation is faster than a restore: