Microsoft is urging customers to keep their Exchange servers updated as well as take steps to bolster the environment, such as enabling Windows Extended Protection and configuring certificate-based signing of PowerShell serialization payloads.
“Attackers looking to exploit unpatched Exchange servers are not going to go away,” the tech giant’s Exchange Team said in a post.
Microsoft also emphasized mitigations issued by the company are only a stopgap solution and that they can “Become insufficient to protect against all variations of an attack,” necessitating that users install necessary security updates to secure the servers.
Another reason is the fact that multiple backend services run as Exchange Server itself, which comes with SYSTEM privileges, and that the exploits could grant the attacker malicious access to the remote PowerShell service, effectively paving the way for the execution of malicious commands.
In some cases, the staging servers used to host the payloads were compromised by Microsoft Exchange servers themselves, suggesting that the same technique may have been applied to expand the scale of the attacks.
The abuse of Microsoft Exchange vulnerabilities has also been a recurring tactic employed by UNC2596, the operators of Cuba ransomware, with one attack leveraging the ProxyNotShell exploit sequence to drop the BUGHATCH downloader.
