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McAfee Labs have been observing a spike in phishing campaigns that utilize Microsoft office macro capabilities. These malicious documents reach victims via mass spam E-mail campaigns and generally invoke urgency, fear, or similar emotions, leading unsuspecting users to promptly open them. The purpose of these spam operations is to deliver malicious payloads to as many people as possible. 

A recent spam campaign was using malicious word document to download and execute the Ursnif trojan. Ursnif is a high-risk trojan designed to record various sensitive information. It typically archives this sensitive data and sends it back to a command-and-control server. 

 

This blog describes how attackers use document properties and a few other techniques to download and execute the Ursnif trojan. 

Threat Summary 

  • The initial attack vector is a phishing email with a Microsoft Word document attachment. 
  • Upon opening the document, VBA executes a malicious shellcode 
  • Shellcode downloads the remote payload, Ursnif, and invokes rundll32.exe to execute it. 

Infection Chain 

The malware arrives through a phishing email containing a Microsoft Word document as an attachment. When the document is opened and macros are enabled, Word downloads a DLL (Ursnif payload). The Ursnif payload is then executed using rundll32.exe 

 

Figure-1: flowchart of infection chain 

Word Analysis 

Macros are disabled by default and the malware authors are aware of this and hence present an image to entice the victims into enabling them.  

Figure-2: Image of what the user sees upon opening the document 

 

VBA Macro Analysis of Word Document 

Analyzing the sample statically with ‘oleId’ and ‘olevba’ indicates the suspicious vectors.. 

 

Figure-3: Oleid output 

 

 

Figure-4: Olevba output 

 

 

 

 

 

 

The VBA Macro is compatible with x32 and x64 architectures and is highly obfuscated as seen in Figure-5 

 

Figure-5: Obfuscated VBA macro 

 

To get a better understanding of the functionality, we have de-obfuscated the contents in the 2 figures shown below. 

 

Figure-6: De-obfuscated VBA macro (stage 1) 

Figure-7: De-obfuscated VBA macro (stage 2) 

 

 

An interesting characteristic of this sample is that some of the strings like CLSID, URL for downloading Ursnif, and environment variables names are stored in custom document properties in reverse. As shown in Figure-7, VBA function “ActiveDocument.CustomDocumentProperties()” is used to retrieve the properties and uses “StrReverse” to reverse the contents. 

We can see the document properties in Figure-8  

    

Figure-8: Document properties 

 

Payload Download and Execution: 

The malicious macro retrieves hidden shellcode from a custom property named “Company” using the “cdec” function that converts the shellcode from string to decimal/hex value and executes it. The shellcode is shown below. 

 

Figure-9: Raw Company property 

 

The shellcode is  written to memory and the access protection is changed to PAGE_EXECUTE_READWRITE. 

 

Figure-10: Code of VirtualProtect 

 

 

Figure-11: Shellcode’s memory and protection after calling VirtualProtect() 

 

After adding the shellcode in memory, the environment variable containing the malicious URL of Ursnif payload is created. This Environment variable will be later used by the shellcode. 

 

Figure-12: Environment variable set in Winword.exe space 

 

 

The shellcode is executed with the use of the SetTimer API. SetTimer creates a timer with the specified time-out value mentioned and notifies a function when the time is elapsed. The 4th parameter used to call SetTimer is the pointer to the shellcode in memory which will be invoked when the mentioned time is elapsed. 

 

Figure-13: SetTimer function (Execution of shellCode) 

 

The shellcode downloads the file from the URL stored in the environmental variable and stores it as ” y9C4A.tmp.dll ” and executes it with rundll32.exe. 

 

URL  hxxp://docmasterpassb.top/kdv/x7t1QUUADWPEIQyxM6DT3vtrornV4uJcP4GvD9vM/ 
CMD  rundll32 “C:UsersuserAppDataLocalTempy9C4A.tmp.dll”,DllRegisterServer 

 

 

Figure-14: Exports of Downloaded DLL 

 

After successful execution of the shellcode, the environment variable is removed. 

 

Figure-15: Removal of Environment Variable 

IOC 

TYPE  VALUE  PRODUCT  DETECTION NAME 
Main Word Document  6cf97570d317b42ef8bfd4ee4df21d217d5f27b73ff236049d70c37c5337909f  McAfee LiveSafe and Total Protection  X97M/Downloader.CJG 
Downloaded dll  41ae907a2bb73794bb2cff40b429e62305847a3e1a95f188b596f1cf925c4547  McAfee LiveSafe and Total Protection  Ursnif-FULJ 
URL to download dll  hxxp://docmasterpassb.top/kdv/x7t1QUUADWPEIQyxM6DT3vtrornV4uJcP4GvD9vM/  WebAdvisor  Blocked 

 

MITRE Attack Framework 

Technique ID  Tactic  Technique Details  Description 
T1566.001  Initial Access  Spear phishing Attachment  Manual execution by user 
T1059.005  Execution  Visual Basic  Malicious VBA macros 
T1218.011  Defense Evasion  Signed binary abuse  Rundll32.exe is used 
T1027  Defense Evasion  Obfuscation techniques  VBA and powershell base64 executions 
T1086  Execution  Powershell execution  PowerShell command abuse 

 

Conclusion 

Macros are disabled by default in Microsoft Office applications, we suggest keeping it that way unless the document is received from a trusted source. The infection chain discussed in the blog is not limited to Word or Excel. Further threats may use other live-off-the-land tools to download its payloads.  

McAfee customers are protected against the malicious files and sites detailed in this blog with McAfee LiveSafe/Total Protection and McAfee Web Advisor. 

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