Ransomware Attackers Bypass Microsoft’s ProxyNotShell Mitigations With Fresh Exploit

The operators of a ransomware strain called Play have developed a new exploit chain for a critical remote code execution (RCE) vulnerability in Exchange Server that Microsoft patched in November.

The new method bypasses mitigations that Microsoft had provided for the exploit chain, meaning organizations that have only implemented those but have not yet applied the patch for it need to do so immediately.

The RCE vulnerability at issue (CVE-2022-41082) is one of two so-called “ProxyNotShell” flaws in Exchange Server versions 2013, 2016, and 2019 that Vietnamese security company GTSC publicly disclosed in November after observing a threat actor exploiting them. The other ProxyNotShell flaw, tracked as CVE-2022-41040, is a server-side request forgery (SSRF) bug that gives attackers a way to elevate privileges on a compromised system.

In the attack that GTSC reported, the threat actor utilized the CVE-2022-41040 SSRF vulnerability to access the Remote PowerShell service and used it to trigger the RCE flaw on affected systems. In response, Microsoft recommended that organizations apply a blocking rule to prevent attackers from accessing the PowerShell remote service through the Autodiscover endpoint on affected systems. The company claimed — and security researchers agreed — that the blocking rule would help prevent known exploit patterns against the ProxyNotShell vulnerabilities.

Novel New Exploit Chain

This week, however, researchers at CrowdStrike said they had observed the threat actors behind Play ransomware use a new method to exploit CVE-2022-41082 that bypasses Microsoft’s mitigation measure for ProxyNotShell.

The method involves the attacker exploiting another

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