Ransomware-wielding attackers are using a new exploit chain that includes one of the ProxyNotShell vulnerabilities to achieve remote code execution on Microsoft Exchange servers.

The ProxyNotShell exploit chain used CVE-2022-41040, a SSRF vulnerability in the Autodiscover endpoint of Microsoft Exchange, while this new one uses CVE-2022-41080 to achieve privilege escalation through Outlook Web Access.

The exploit chain – dubbed OWASSRF by Crowdstrike researchers – can only be headed off by implementing the patches for Microsoft Exchange released in November 2022.

The researchers spotted in-the-wild exploitation of CVE-2022-41082 when investigating Play ransomware intrusions where the common entry vector was Microsoft Exchange.

In the meantime, Huntress Labs threat researcher Dray Agha managed to grab attack tools via an open repository and among them was a PoC script leveraging an unknown OWA exploit technique and the CVE-2022-41082 exploit.

“After initial access via this new exploit method, the threat actor leveraged legitimate Plink and AnyDesk executables to maintain access, and performed anti-forensics techniques on the Microsoft Exchange server in an attempt to hide their activity,” Crowdstrike researchers concluded, and provided additional advice for mitigating the risk and detecting signs of exploitation.