The Ursnif sample comes from a Microsoft Word document containing a malicious VBA macro. The document is straightforward, simply displaying an image that asks the user to enable macros.
If macros are already permitted, the macro is executed automatically when opening the document via the AutoOpen function. The macro is mostly obfuscated code that executes math functions on data that does not relate to the next stage.
There is only one line in the macro that is important to executing the next stage, ultimately executing PowerShell.
The value of this property is the malicious PowerShell command, which is subsequently executed by the Shell function. The PowerShell command is base64 encoded, and is another PowerShell command that downloads Ursnif. Specifically, it downloads an executable from its C2 to the AppData directory and executes it.
Note, this is where the Exploit Prevention engine stops executing the downloaded file and provides us with alerts to investigate.
Malicious VBA Macro
- The malicious VBA macro, if enabled, will automatically run using AutoOpen.
- Otherwise, the malicious document displays an image prompting users to enable the macros feature.
- The malicious VBA macro contains a single line which is important to execute the next infection stage by executing PowerShell.
- This single line accesses the AlternativeText property of the Shapes object ‘j6h1cf’.
- The value of this property is the malicious PowerShell command, which is eventually executed by the Shell function.
- The malicious Powershell command is base64 encoded and it is the one that downloads the Ursnif executable from its C&C server to the AppData directory and executes it.
- Once the Ursnif executable is downloaded and executed, registry data is created for the next stage of infection.
- The PowerShell command for the next stage of infection exists in the value of the APHohema key.
- This PowerShell command uses Windows Management Instrumentation Command-line (WMIC) to execute PowerShell, which extracts the value of the Authicap key to execute it.
- Ursnif is a fan of “fileless” persistence which makes it difficult for traditional anti-virus techniques to filter out the C2 traffic from normal traffic. Additionally, Ursnif uses CAB files to compress its data prior to exfiltration, which makes this malware even more challenging to stop. To help with the detection of this malware, we are providing readers with a list of IOCs below that can help you stop Ursnif before it infects your network.
C2 Server Domains: IOCs