Security researchers observed that a new Ursnif attack campaign replaced PowerShell with mshta as a means to distribute the malware.
Zscaler observed that the Ursnif campaign began with the delivery of document files bearing the name “info_03_24.doc.” These documents leveraged malicious Visual Basic for Applications (VBA) macro code to call the main routine. This stage involved writing the second-stage payload to “index.html” and executing it.
In contrast to previous campaigns, the second stage of the campaign did not invoke a PowerShell command. Instead, it executed index.html using “mshta.exe,” a utility for executing Microsoft HTML Applications (HTAs). The step executed JavaScript and ActiveX code that created a new function with decoded ASCII data as its function body, among other operations.
The third and final stage leveraged that decoded ASCII data’s instructions to execute and download “index.dll” via regsvr32. In so doing, it installed Ursnif as the campaign’s final payload.
Ursnif’s Fork and Globetrotting Campaigns
Ursnif has been featured in several attack campaigns thus far in 2020. In January, for instance, researchers at FireEye detected malware that identified itself as “SaiGon version 3.50 rev 132.” A closer look revealed that this threat had based itself on the source code of Ursnif v3, suggesting a possible fork in the malware family’s development.
Just a few weeks later, SANS ISC unveiled its discovery of an attack campaign that relied on malspam to target German users with malware. Then, in March 2020, Cybaze-Yoroi Zlab intercepted a campaign in which attackers used a compromised Italian website to target Italian users with the Trojan.
How to Defend Against Malicious Macros
Security professionals can help defend their organizations against campaigns that use malicious macro code by relying on their security information and event management (SIEM) solution to detect malicious macro activity. Specifically, they should use the SIEM to detect the creation of new processes that could be spawned by malicious macros. Infosec personnel should also use tools like a VBA editor to extract and inspect macro code included in suspicious Office documents.