When it comes to malware and cybercriminals, sometimes "cheap" and "fast" clearly trumps "tested" and "sophisticated."
That's the case with Rubella Macro Builder, a recently discovered crimeware kit that, despite being new and relatively unsophisticated, has been gaining popularity among cybercriminals - including members of the suspected Russian gang behind the Panda banking malware.
Security vendor Flashpoint, which issued an advisory on the threat this week, described Rubella as enabling criminals to generate Microsoft Word (.doc) and Microsoft Excel (.xls) payloads. "The Rubella-generated malware acts as a first-stage loader for other subsequent malware downloads and installations on targeted machines," says Vitali Kremez, director of research for Flashpoint.
Since it surfaced in February, Rubella Macro Builder has been used as a first-stage loader in one of the most recent Panda campaigns. The group behind the distribution appears to have targeted victims through various social media platforms and through webinjects, Flashpoint says.
"Flashpoint identified Rubella malware infection leading to the execution of the Panda banking malware version 2.6.6 and Gootkit banking malware," Kremez says. Panda and Gootkit are designed to harvest credentials, infect browsers through webinjects and enable remote PC access via a hidden virtual network-computing module, he says.
When it first surfaced in February, the authors of the Rubella crimeware kit priced it at a relatively low $500 per month. Since then, prices for the kit have dropped even further to just around $150 for a three-month subscription even as it has acquired several new capabilities, according to FlashPoint.
Rubella Macro Builder does not exploit any security vulnerabilities. Rather, it relies on social engineering techniques to force victims to enable malicious macro execution to run it, Kremez says. It is typically distributed to intended victims via Microsoft Word or Excel email attachments during spam campaigns. It comes with some rudimentary but nevertheless effective obfuscation methods for bypassing basic AV tools.
"The macro builder has a clear appeal for cybercriminals: it’s cheap, fast, and can defeat basic static anti-virus detection," Kremez says.
Rubella is somewhat similar to ThreadKit, a more advanced malware kit that researchers at Proofpoint discovered last October and described in an alert in March. ThreadKit, like Rubella is a Microsoft Office document exploit builder, but packs more features, including a mechanism for reporting infection statistics back to the operators of the malware.
ThreadKit has been used quite extensively to spread numerous malware payloads including Trickbot and remote access Trojans like Loki Bot and FormBook. One well-known crime group that has been using the kit is the Cobalt Gang, a threat actor associated with various ATM heists.
"Microsoft Office macro-based malware appears to still be threat actors' preferred method for obtaining initial access to compromised machines," Flashpoint said in its advisory. Microsoft Office-based loader malware like Rubella work well as an intial decoy because they look like commonly exchanged Word and Excel documents and attachments.
According to the security vendor, Rubella Macro Builder represents a moderate threat for enterpises given its ability to beat static AV tools and its low pricing model. To mitigate their exposure to the threat, organizations should pay attention to email messages with suspicious Word or Excel attachments, especially those that ask permission to 'Enable Content' for running macros, FlashPoint said.
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