Analyzing attacks that exploit the CVE-2021-40444 MSHTML vulnerability

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In August, Microsoft Threat Intelligence Center (MSTIC) identified a small number of attacks (less than 10) that attempted to exploit a remote code execution vulnerability in MSHTML using specially crafted Microsoft Office documents. These attacks used the vulnerability, tracked as CVE-2021-40444, as part of an initial access campaign that distributed custom Cobalt Strike Beacon loaders. These loaders communicated with an infrastructure that Microsoft associates with multiple cybercriminal campaigns, including human-operated ransomware.

The observed attack vector relies on a malicious ActiveX control that could be loaded by the browser rendering engine using a malicious Office document. Customers who enabled attack surface reduction rules to block Office from creating child processes are not impacted by the exploitation technique used in these attacks. While these attacks used a vulnerability to access entry point devices and run highly-privileged code, the secondary actions taken by the attackers still rely on stealing credentials and moving laterally to cause organization-wide impact. This illustrates the importance of investing in attack surface reduction, credential

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