Microsoft SmartScreen Zero-Day Exploited to Deliver Magniber Ransomware

A cybercrime group has been exploiting a zero-day vulnerability in the Microsoft SmartScreen security feature to deliver the Magniber ransomware, Google warned on Tuesday.

Google’s Threat Analysis Group (TAG) said the vulnerability, tracked as CVE-2023-24880, has been exploited since at least January. The internet giant’s researchers reported their findings to Microsoft on February 15 and a fix has been released with Microsoft’s latest Patch Tuesday updates. 

The SmartScreen feature is designed to protect users against phishing and malware, including by flagging potentially malicious files downloaded from the web. 

The financially motivated threat actor that is behind the Magniber ransomware has exploited CVE-2023-24880 to deliver specially crafted MSI files. These files are signed with an Authenticode signature that is invalid but crafted in a way that causes SmartScreen to return an error. 

This results in users not being shown an alert associated with the Mark-of-the-Web (MotW) security feature, which is used to prevent the execution of potentially malicious files originating from the internet. 

The Magniber ransomware operation emerged in 2017 and until now it has mainly targeted South Korea and Taiwan. However, in the recent attacks observed by Google, over 80% of the more than 100,000 malicious MSI file downloads have been associated with users in Europe.

While these attacks bypassed SmartScreen, Google said its Safe Browsing mechanism did warn users in more than 90% of cases.  

This is not the first time the Magniber hacker group has exploited a SmartScreen bypass technique. In fact, CVE-2023-24880 is a new variant of CVE-2022-44698, a vulnerability that was previously exploited by the cybercriminals. 

CVE-2022-44698 has been exploited by the group to deliver malicious JScript files since at least September 2022. The same method was later used by a different cybercrime group to distribute the Qakbot malware. Microsoft only released a patch in December 2022. 

“This security bypass is an example of a larger trend Project Zero has highlighted previously: vendors often release narrow patches, creating an opportunity for attackers to iterate and discover new variants,” Google said in a blog post on Tuesday. 

“When patching a security issue, there is tension between a localized, reliable fix, and a potentially harder fix of the underlying root cause issue. Because the root cause behind the SmartScreen security bypass was not addressed, the attackers were able to quickly identify a different variant of the original bug,” it added.

Google has made available technical details for both CVE-2023-24880 and CVE-2022-44698, as well as indicators of compromise (IoCs) for the Magniber and Qakbot attacks. 

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