Security researchers are warning of previously undisclosed flaws in fully patched Microsoft Exchange servers being exploited by malicious actors in real-world attacks to achieve remote code execution on affected systems.

“We detected webshells, mostly obfuscated, being dropped to Exchange servers,” the company noted.

“Using the user-agent, we detected that the attacker uses Antsword, an active Chinese-based open source cross-platform website administration tool that supports web shell management.”

Html Exchange Server vulnerabilities, with GTSC noting that the targeted servers had already been patched against the flaws that came to light in March 2021.

It’s worth noting that the China Chopper web shell was also deployed by Hafnium, a suspected state-sponsored group operating out of China, when the ProxyShell vulnerabilities were subjected to widespread exploitation last year.

“If you don’t run Microsoft Exchange on premise, and don’t have Outlook Web App facing the internet, you are not impacted,” Beaumont said.