"A range of threat actors have been targeting exploitable servers with a variety of malware, from webshells to ransomware," he wrote.
"But those aren’t the only payloads we’ve seen directed at Exchange servers: An unknown attacker has been attempting to leverage what’s now known as the ProxyLogon exploit to foist a malicious monero cryptominer onto Exchange servers, with the payload being hosted on a compromised Exchange server."
The zero-day exploits were made public on 3 March, with the company saying that attackers would be able access email accounts and also allow installation of malware to ensure long-term residence in victims' systems.
Brandt said the attack began with a PowerShell command to get hold of a file named win_r.zip on another compromised server's Outlook Web Access logon path (/oaw/auth).
This file with a .zip extension was not a compressed file, but rather a batch script that called the Windows certutil.exe program to pull in two more files which were also posing as compressed archives.
"The first file is written out to the filesystem as QuickCPU.b64," Brandt noted. "The certutil application is designed to be able to decode base64-encoded security certificates, so the attackers have leveraged that functionality by encoding an executable payload in base64 and wrapping it in headers that indicate it is some form of digital certificate."
He said the batch script then put the decoded executable into the same directory and the miner and configuration data were then extracted, injected into a system process and then deleted.
"The file uses forged data in its Properties sheet that indicates the file is a Windows component, but the binary is not digitally signed and, besides, no such file has ever existed as a standard component of Windows, though there is a legitimate utility with the same name, made by a third-party software developer," Brandt wrote. "That utility is not connected to this malware in any way."
The executable appeared to have a doctored version of a tool available on GitHub which could migrate any x64 exe to any x64 process without having admin privileges.
"When it runs, it extracts the contents of the QuickCPU.dat file (an installer for the miner, and its configuration) temporarily to the filesystem, configures the miner, injects it into a running process, then quits," Brandt explained.
"The batch file then deletes the evidence and the miner remains running in memory, injected into a process already running on the system."
The wallet denoted in the miner had begun receiving funds on 9 March, he said, adding that some servers that were infected had been lost, but others gained.