Google’s Threat Analysis Group (TAG) recently discovered usage of an unpatched security bypass in Microsoft’s SmartScreen security feature, which financially motivated actors are using to deliver the Magniber ransomware without any security warnings. The attackers are delivering MSI files signed with an invalid but specially crafted Authenticode signature. The malformed signature causes SmartScreen to return an error that results in bypassing the security warning dialog displayed to users when an untrusted file contains a Mark-of-the-Web (MotW), which indicates a potentially malicious file has been downloaded from the internet.
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Source: Threat Analysis Group