The initial infiltration was accomplished by leveraging the ProxyShell exploit in Microsoft Exchange servers to allow an unauthenticated attacker to execute arbitrary commands on them through an exposed HTTPS port.
As a result, the attackers were able to deploy two web shells:
C:/inetpub/wwwroot/aspnet_client/system_web/iispool.aspx
C:/inetpub/wwwroot/aspnet_client/system_web/map.aspx
These two web shells are used in conjunction with one another, and some of their functionalities overlap.
On numerous occasions, map.aspx was used to validate the results of the commands executed by iispool.aspx.
Post infection, the attackers dedicated several days to the exfiltration of PST files and other sensitive data from the compromised server.
Next, they attempted to steal credentials by creating a memory dump of lsass.exe using a LOLBin.
Finally, the attackers dropped and installed the backdoor components.