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Criminals behind the Ryuk ransomware were early exploiters of the Windows MSHTML flaw, actively leveraging the bug in campaigns ahead of a patch released by Microsoft this week. Collaborative research by Microsoft and RiskIQ revealed campaigns by Ryuk threat actors early on that exploited the flaw, tracked as CVE-2021-40444. The bug is a remote code execution (RCE) vulnerability in Windows that allows attackers to craft malicious Microsoft Office documents. The two released separate reports online this week to provide a look into who has been using the flaw–which can be used to hide a malicious ActiveX control in an Office document–in attacks, as well as their potential connections to known criminal groups. Specifically, most of the attacks that researchers analyzed used MSHTML as part of an initial access campaign that distributed custom Cobalt Strike Beacon loaders, which communicated with an infrastructure that is associated with multiple cybercriminal campaigns–including human-operated ransomware, researchers from the Microsoft 365 Defender Threat Intelligence Team at the Microsoft Threat Intelligence Center (MSTIC) reported. RiskIQ identified the ransomware infrastructure as potentially belonging to the Russian-speaking Wizard Spider crime syndicate, known to maintain and distribute Ryuk ransomware. “Based on multiple overlapping patterns in network infrastructure setup and use, we assess with high confidence that the operators behind the zero-day campaign are using…

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